Science Helpdesk

The Universe is about 14 billion years old, and is formed by more than 100 billion galaxies. A galaxy is a huge system of stars, interstellar gas and dust. Typical galaxies contain from ten million to one trillion stars, all orbiting a common centre of gravity. Some galaxies are elliptical shape, some are spiral, others are irregular. Galaxies are usually separated from others by distances on the order of millions of light years. Groups of galaxies gravitationally attracted between themselves are called galactic clusters.

The Solar System is located in the Milky Way galaxy, a spiral galaxy with a diameter estimated at about 100,000 light years, containing approximately 200 billion stars. The Milky Way belongs in a cluster of over 30 galaxies known as the Local Group. The galaxy of Andromeda is the nearest to the Milky Way and the biggest one in the Local Group.

The Solar System resides in one of the Milky Way's spiral arms, known as the Orion Arm, at about 27,000 light years from the galactic centre. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years.

Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. Some stars are alone in the sky (the Sun, the North Star), others have companions (Sirius, which is a binary star system, and Alpha Centauri, which is a ternary star system). The nearest star to the Sun is Alpha Centauri C or Proxima Centauri. The brightest star in the Northern hemisphere's night sky is Sirius.

Not all stars are the same: stars come in all sizes, brightnesses, temperatures and colours:

• The colour of a star is due to its temperature. A blue or white star is hotter than a yellow star, which is hotter than a red star.
• The brightness of a star depends on two factors: (a) its distance from us and (b) its luminosity: how much energy it puts out in a given time, which, in turn, depends on its size. Think on a street lamp, which puts out more light than a hand torch. That is, the street lamp is more luminous. But if that street lamp is 1 km away from you, it will not be as bright, because light intensity decreases with distance. In this case, a hand torch held 10 m away from you would be brighter than the street lamp. The same is true for stars.

Constellations are groups of stars visibly related to each other in a particular pattern. Some well known constellations contain familiar patterns of bright stars. Examples are Ursa Major (containing the Big Dipper or Plough), Orion (that resembles the figure of a hunter) and Cassiopeia (with the shape of a "W"). The stars of a constellation, although appearing to be very near, may be millions LY away one to another.

The Solar System is the stellar system formed by the Sun and the group of celestial objects gravitationally bound to it:

• the eight planets and their 162 known moons,
• five dwarf planets and their six known moons, and
• thousands of small solar system bodies (SSSB).

The Sun is the main component of the Solar System, a star that contains 99.9% of the Solar System's mass. The Sun releases enormous amounts of energy in the form of electromagnetic radiation, which includes visible radiation (light), ultraviolet radiation and infrared radiation.

The planets are the biggest objects orbiting the Sun. Their orbit is almost circular. In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky planets; the four outer planets are gaseous giant planets with a small rocky core. All planets but the two first are orbited by natural satellites (usually called "moons"). The planets, with the exception of Earth, are named after gods and goddesses from Greco-Roman mythology. The following table shows some major magnitudes measured relative to the Earth:

The dwarf planets are also rocky objects orbiting the Sun, smaller than the planets, but bigger than asteroids. There are currently (20-sep-2008) five dwarf planets in the Solar System; the two best known of which are:

• Ceres, formerly considered the Solar System's biggest asteroid;
• Pluto, a trans-neptunian object formerly considered the Solar System's ninth planet;

The SSSBs comprise several types of celestial bodies, the best known of which are:

• Asteroids = planetoids = minor planets. They're the smallest rocky bodies orbiting the Sun. Unlike planets and dwarf planets, they are not spherical, but irregularly shaped. Most of them occupy orbits between the ones of Mars and Jupiter, and make up the asteroid belt. The biggest asteroid is called Vesta.
• Comets, very small icy trans-neptunian objects that orbit the Sun in very eccentric orbits. When a comet approaches the Sun, its icy surface begins to boil away, creating two long tails, one of gas and another of dust, which are often visible with the naked eye. Two well known comets are Halley and Hale-Bopp.

The International System of Units (abbreviated SI from the French language name Système international d'unités) is the world's most widely used system of units. The SI was developed in 1960 and, with few exceptions, is used in every country in the world.

The SI base units for the seven primary quantities are:

Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of electric current is named after André-Marie Ampère, so its symbol is written "A", whereas the unit itself is written "ampere". The only exception is the litre, whose original symbol "l" is unsuitably similar to the numeral "1"; thus it is recommended that "L" be used instead.

Abbreviated symbols should not be pluralized: for example "25 kg", not "25 kgs".

Symbols do not have an appended period (.) unless at the end of a sentence.

A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth. The SI main prefixes are as follows:

Quantities are the measurable properties of the physical bodies. Some of the most important ones are the following:

Mass	Is the amount of matter in a body. Its unit in the SI is the kg.
Volume	Tells how much space an object occupies. Its unit in the SI is the m3.
Capacity	Is the amount of space that can be contained by a body. It is measured in L.
Density	Expresses how concentrated is the matter in a body, this is, how much matter is there in a given unit of volume in a body. It is always the same for the same type of substance, this is, it doesn't depend on the size of the object. Its unit in the SI is the kg/m3.
Temperature	It is not a kind of energy but a measure of the amount of heat in a body. It depends on the movement of its particles: the quicker the movement, the higher the temperature. Its unit in the SI is the K, but is commonly expressed in °C or °F.

What is it?

The water cycle, also known as the hydrological cycle, is the circulation of water between the different compartments or reservoirs of the Earth's Hydrosphere, involving changes in the physical state of water between liquid, solid, and gaseous phases. The water cycle is powered by the Sun's energy and the Earth's gravity.

The Earth's water cycle involves the following main physical processes:

Evaporation	Is the transfer of water from bodies of surface water into the atmosphere. This transfer involves a change in the physical state of water from liquid to gaseous phases, powered mainly by the solar radiation. 90% of atmospheric water comes from evaporation.
Evapotranspiration	Is the transfer of water from living beings into the atmosphere. This transfer involves a change in the physical state of water from liquid to gaseous phases, powered mainly by the solar radiation and the heat released by the metabolism of the living beings. 10% of atmospheric water comes from evapotranspiration.
Condensation	It takes place when water vapour in the air accumulates to form liquid water droplets in clouds and fog.
Precipitation	Is atmospheric moisture that has previously condensed (or solidified), falling to the surface of the Earth. This happens mostly as rainfall, but also as snow, hail, or fog.
Surface runoff	Includes the variety of ways by which land surface water moves down slope to the oceans: snowmelt runoff to streams, streamflow, riverflow… Water flowing in streams and rivers may be delayed for a time in lakes. Much of the precipitated water evaporates before reaching the ocean or infiltrates into the soil.
Infiltration	Is the transition of land surface water into the ground. The infiltration rate depends on soil or rock permeability. Infiltrated water may become part of the soil moisture or accumulate in aquifers: in this case it is called groundwater.
Groundwater flow	Includes the movement of groundwater in aquifers. Aquifers tend to move slowly, so the water may return as surface water (into rivers, lagoons, oceans or through springs) after thousands of years in some cases. Water returns to the land surface at lower elevation than where it infiltrated.
Absorption or drinking	Are the ways in which soil moisture or surface water is taken in by living beings.

Volume of water stored in the water cycle's reservoirs:

	Volume (106 km3)	Percent of total
Seas and oceans	1370	97.25
Ice caps, glaciers and snow covers	29	2
Groundwater	9.5	0.7
Lakes	0.125	0.01
Soil moisture	0.065	0.005
Atmosphere	0.013	0.001
Streams and rivers	0.0017	0.0001
Living beings	0.0006	0.00004

Average reservoir residence times:

Groundwater: deep	10,000 years
Seas and oceans	3,200 years
Groundwater: shallow	100 to 200 years
Lakes	50 to 100 years
Ice caps and glaciers	20 to 100 years
Streams and rivers	2 to 6 months
Seasonal snow covers	2 to 6 months
Soil moisture	1 to 2 months
Atmosphere	9 days

Sedimentary rocks can be made up of clasts or detritus (rock grains) weathered from other rocks by river waters, wind, coastal sea waters, glaciers, living beings, etc. This kind of sedimentary rocks are called detrital rocks.

They're classified upon the grains that they are made up of. First up, grains may be all the same size, uniform, as in a sandstone, or different sizes, as in a conglomerate. Secondly, grains may be bigger or smaller. The following table shows roughly how rock grains are classified upon their size:

	Diameter
Clay	< 0.004 mm
Silt	< 0.06 mm
Sand	< 2 mm
Gravel	< 6 cm
Cobble	< 25 cm
Boulders	> 25 cm

The term lustre refers to the appearance of a mineral surface in reflected light. The main types of mineral lustre are the following:

Lustre	Opacity	Examples
Adamantine	Transparent	Diamond
Vitreous	Translucent	Quartz, Halite, Olivine
Wet	Translucent	Fluorite
Resinous	Translucent	Yellow and red sphalerite varieties
Wax-like	Translucent	Talc
Greasy	Variable	Milky quartz
Silky	Variable	Fibrous gypsum
Pearly	Poorly translucent	Orthoclase, Mica aggregates
Dull metallic	Opaque	Cinnabar aggregates
Metallic	Opaque	Pyrite, Chalcopyrite, Magnetite

This is not the most modern way of classification of living beings, but still proves to be quite useful to understand and organise the huge diversity of the living beings:

A force is a push, a pull or a twist which is exerted in an specific direction. Forces are measured with balances. The small ones can be measured with a spring balance (= forcemeter). Their values are expressed in newtons (N).

Forces can have several effects upon the objects: they can (a) change their shape, (b) change their direction if they're moving; and (c) change their speed by slowing them down, speeding them up or putting them into movement.

The movement of an object in any medium (except the vacuum) produces automatically a force in exactly the opposite direction: this is the force of friction (of the air, the water, the ground). This force slows down the moving object and tends to ultimately stop it. It requires the moving object to use more energy to overcome the friction and reach the wanted speed.

But the force of friction doesn't only imply a higher use of energy: it can also be useful. For instance: it is the force of friction generated when you push your feet backwards against the ground, the one that, in return, pushes you forward when you are trying to run. And it is the force of friction done by a rubber pad against a wheel the one that makes a bike or a car slow down when you press the brake.

When two forces are acting on an object in opposite directions they can...

• Be equal in size: these are balanced forces and cancel each other out, i.e., the resultant force is zero. One example is when the force of friction of the air and the ground (called drag) cancel out the driving force of a car (produced by the engine thrust). In this case the car moves at a steady speed, called terminal speed. Another example is when you are standing still on the floor: you know that the force of gravity is always pulling you downwards, so to keep you there, instead of being swallowed by the Earth, the floor must be doing a force upon you exactly the same size than the gravity force, but in the opposite direction.
• Be different in size: these are unbalanced forces and do not cancel each other out. They change the motion of the object, which (a) will start to move in the direction of the bigger force, if the object was still; (b) will move faster, if the object was moving in the direction of the bigger force; (c) will slow down, if the object was moving opposite to the bigger force.

Gravity is a force. It is the force that pulls objects together. It is bigger the bigger and the closer these objects are. The gravity force between two small objects or between two very distant stars isn't perceptible, but it is clearly noticeable between the Earth and every object on its surface, because every object has a weight, and this is the force with which the Earth pulls any near object downwards.

Therefore, your weight will be bigger (a) the closer you are to the centre of gravity of the Earth (which is the centre of the Earth) and (b) the bigger you are, or more exactly, the more mass you have. Your mass is the amount of matter you have (atoms and molecules), and with the same mass you can have different weights: your weight will be smaller in the stratosphere, because you are further away of the Earth's centre of gravity, or in Mars, because it has about 8 times less mass than the Earth: in both cases the gravity acting on you is smaller than in the surface of the Earth. But your weight would be bigger in Saturn or Jupiter, for the opposite reason: there is a bigger gravity acting on you now.

As weight is a force, it is measured in newtons. In the Earth, at sea level, the numeric value of your weight, in newtons, is roughly ten times your mass. In the Moon, you have to multiply your mass by only 1.7 to find how many newtons you weight. And in the outer space, far away from the gravitational action of any celestial body, your weight would be almost zero. In the three cases you would have exactly the same mass.

Speed is the rate at which an object covers a distance. You calculate it dividing the space covered by an object, by the time taken on it. Its unit in the SI is m/s.

Distance-time graphs are useful to represent the variation of the distance covered by an object in every unit of time, i. e., they are useful to represent visually the speed of an object. When the line in the graph shows a steady slope (when it is a straight line), the object has a steady speed; otherwise, the graph will indicate some sort of positive or negative acceleration. Also, a steep slope will express a higher speed than a gentle slope. And a straight horizontal line will show that the object is stopped.

If you plot a speed-time graph you will represent visually the acceleration of an object. In this case, a straight horizontal line shows acceleration zero, i. e., the object is moving at a steady speed. A straight line with a positive slope will represent an object speeding up. And a straight line with a negative slope will represent a negative acceleration: the object is slowing down. If you want to represent a stopped object in this kind of graph, you have to draw a straight line along the X axis.

The goal of power plants is to produce electricity that is later dumped into the general supply network. That electricity is produced by an electrical generator driven by a turbine, in all cases. The differences between the several types of power plants come with the way in which that turbine is moved:

• In the hydroelectric power stations, it is the water flow of a river what moves the turbine.
• In an aerogenerator it is the wind what moves the turbine (which consists of a set of three big blades).
• In the rest of the cases it is water vapour at a very high pressure what moves the turbine. That water vapour is produced by heating water, and the heat can come from…
• igniting coal or other fossil fuels or biomass (in thermal power plants);
• breaking nuclei of uranium or plutonium (in nuclear power plants).

This way, the production of heat in a power plant goes through a series of stages in which a transfer and a transformation of energy take place. The following diagram summarizes what happens in a coal-fueled thermal power plant:

• Chemical energy in the coal -> Thermal energy in the water vapour -> Kinetic energy in the turbine -> Electric energy in the generator and to the supply network wires

Physical changes are those in which the substances that make up a physical body remain the same when the body undergoes a change.

A physical change can be produced…

• When you apply a force to an object, as when you crumble a piece of paper with your hand;
• When you give or take energy to/from an object, as when you heat up or cool down water.

Some examples of physical changes are…

• A change of state, such as the condensation of water;
• A change of temperature, as the cooling down of food inside the fridge;
• A change in the shape, as when you shatter mom's favourite china vase;
• A change in the amount or type of energy in an object: so happens when you drop a stone to the floor.

Chemical changes (or chemical reactions) are those that transform some chemical/s into new other chemical/s.

They normally occur when different substances, prone to react between them, are put into contact. And so, when iron (Fe) and oxygen (O₂) are put together, they will combine and disappear to form a new chemical: rust (Fe₂O₃). But the wine and the nitrogen (kept in the upper part of the bottles of wine) will never react, allowing the wine to preserve its properties over time. Not all chemicals will react when put into contact.

The initial molecules that react in a chemical reaction are called reactants, and the final resulting molecules are called products. In the previous reaction, the oxygen and the iron are the reactants, whereas the rust is the only product.

Some chemical changes imply a loss of energy, i.e., they are exothermic, because they release energy (usually heat) to the environment. Some other chemical changes are endothermic, they absorb energy from the environment, and the products will have a greater amount of energy than the reactants.

Nevertheless, most chemical reactions, either exothermic or endothermic, require an initial contribution of external energy to take place. If you want a piece of paper to combine with the atmospheric oxygen and go into combustion, you need to heat up the paper (what you usually do with a flame). But during the reaction, the paper releases a lot of energy (heat, light and even sound), so the reaction is exothermic: the paper and the oxygen had more energy than the ashes, the smoke, the CO₂ and the water vapour produced.

Quite often, chemical changes can be noticed by some conspicuous events, such as…

• A change in the temperature, as in any combustion;
• A change in the colour, as in the rusting of iron;
• The formation of bubbles, as when the calcite (CaCO₃) reacts with hydrochloric acid (HCl);
• A change in the volume, as in baking bread;
• Etc.

Most chemical changes are irreversible: they can't be undone (e.g. a combustion), whereas most physical changes are reversible (e.g. the condensation of water).

Chemical equations are the way by which chemical reactions are represented. To the left you write the formulas of the reactants (either elements, molecules or crystals) and to the right, the formulas of the products. In between, you draw an arrow that represents the direction of the change: from the reactants to the products. In case of a reversible reaction, you must draw a double arrow.

One example would be the following:

• 3 H₂ + N₂ → 2 NH₃

The number of molecules of each type is represented with a coefficient to the left of the molecule. The number of atoms for each kind of molecule is represented by a subindex to the right of its chemical symbol (nothing when it's just one). Thus, you can see that in the equation above we have six atoms of hydrogen and two of nitrogen in each side. This is because in a chemical reaction, neither the chemical elements, nor the number of atoms of each chemical element change. What changes is the arrangement of those atoms because a chemical change actually consists of the breaking down of some chemical bonds and the formation of some new chemical bonds.

The following chemical equation does not represent any real chemical reaction, because there aren't exactly the same atoms in each side of the equation:

• H₂ + N₂ → NH₃

Also, as the atoms of the reactants have to be the same as the atoms of the products, the overall mass does not change during a chemical reaction.

Balancing a chemical equation means writing the right coefficients to the right of each molecular species, to ensure that the number of each type of atoms (and so, the overall mass) remains the same along the equation.

Metals with oxygen and water

Metals react with oxygen and water depending on their reactivity. This means that some are highly reactive, some others are moderately reactive, and some others are very little reactive or even nothing at all. The following list shows the reactivity of some common metals: K > Na > Ca > Mg > Al > Zn > Fe > Sn > Pb > Cu > Ag > Au > Pt.

This way, while potassium reacts violently with O₂, iron forms rust (Fe₂O₃), silver slowly tarnishes (forming Ag₂O), and gold does not react at all and preserves its properties forever.

Likewise, as combustions are oxidations boosted by heat, the behaviour of metals when they are heated varies: some burn (Mg) while others just rust (Cu).

Metals with acids

Metals react with acids forming hydrogen gas (H₂) and a salt. This reaction consists of the displacement of the hydrogen of the gas by the metal, as this one is more reactive than the hydrogen.

One example is sodium reacting with hydrochloric acid to form sodium chloride (table salt) and hydrogen:

• Na + HCl → NaCl + H₂

This type of reactions can be tested by testing the formation of H₂, which is a gas that is able to put out the flame of a match.

Displacement of metals by metals

As some metals are more reactive than others, the more reactive metals can displace the less reactive metals from their compounds. For instance: magnesium is more reactive than iron, and so it can displace the iron from the ferric oxide:

• 3 Mg + Fe₂O₃ → 3 MgO + 2 Fe

For the same reason, platinum would never displace any other metal from its compounds, as it is the least reactive metal of all.

These type of reactions are useful to purify metals that are rarely found in pure state in nature. This is the case of iron, that usually occurs forming the minerals hematite (Fe₂O₃) and magnetite (Fe₃O₄). Aluminium can be used to displace iron from its oxides, and thus, purify it:

• 2 Al + Fe₂O₃ → Al₂O₃ + 2 Fe

Acids and alkalis

Acids and alkalis are common everyday substances, such as acetic acid (in vinegar), citric acid (in lemon juice), caustic soda (lye, NaOH) or bicarbonate of soda (baking soda).

Acids and alkalis have a very different behaviour in solution: while acids release protons (H⁺), alkalis sequestrate the protons. The amount of free protons is measured with the pH scale: the greater the amount of free protons, the lower the pH, and the lower the amount of free protons, the higher the pH. Very acidic solutions have a pH of 1, whilst very alkaline solutions have a pH of 14. Neutral solutions and distilled water, neither acidic nor alkaline, have a pH of 7. Salts make neutral solutions; such is the case of a solution of table salt in water.

Indicators, such as litmus paper or a solution of phenolphthalein, can tell us whether a solution is acidic or alkaline: litmus paper or litmus solution will turn red in an acidic solution, and blue in an alkaline solution. Universal indicator is more precise, though: it is a mixture of dyes that can be present in paper strips or in solutions, and it yields a wide range of colours for the whole pH scale: from red (very acidic solutions) to green (neutral solutions) and to purple (very alkaline solutions).

Reactions between acids and bases

Alkalis are soluble substances that belong to a group of chemicals called bases. Bases react with acids producing a salt and water. These are called neutralisation reactions, as both the acid and the base "disappear" to form a salt, which gives a neutral solution

Some examples of everyday neutralisation reactions are the following:

• Treating bee stings (acidic) with sodium bicarbonate (a base);
• Treating wasp stings (alkaline) with vinegar (that contains acetic acid);
• Upping the pH of an acidic soil with lime (CaOH, an alkali);
• Neutralising the excess of HCl that your stomach produces during digestion with antacids such as baking soda (a base).

The reactions between acids and bases can produce a wide range of different salts. For example:

• Salts of sulphuric acid are called sulphates:
•     H₂SO₄ (sulphuric acid) + 2 NaOH (caustic soda) → Na₂SO₄ (sodium sulphate) + 2 H₂O

• Salts of nitric acid are called nitrates:
•     HNO₃ (nitric acid) + NaOH (caustic soda) → NaNO₃ (sodium nitrate) + H₂O

• Salts of hydrochloric acid are called chlorides:
•     2 HCl (hydrochloric acid) + CaCO₃ (calcium carbonate) → CaCl₂ (calcium chloride) + H₂O + CO₂

Living matter is the one able to carry out the three so called vital functions:

• Nutrition, which consists of taking in matter and energy in order to grow, survive and reproduce; waste matter and waste energy are produced as by-products. If a living being feeds directly off other living beings (such as animals, fungi and protozoa) it is called a consumer or heterotroph; but if it takes the matter and the energy that it needs from the inert matter (such as plants and algae) then it is a consumer or autotroph. Another way to express this difference is that consumers take in both organic and inorganic molecules, while the producers only feed on inorganic substances.
• Interaction is (a) the ability to perceive what is going on in both the environment and the inside of the organism itself, and (b) the ability to produce responses coherent with the information that was perceived. It usually goes as follows: an stimulus is perceived by a receptor → a control centre analyses the stimulus and generates a response order → an effector performs the response.
• Reproduction, the ability to produce living beings similar to the parental organisms. It may be sexual (when there is only one parental organism, as in bacteria) or sexual (when two different types of individuals, male and female, are required).

Every living being, and every cell in the multicellular living beings, is able to carry out these three vital functions.

The Ecosystems

One ecosystem is any area that happens to have an specific set of features (living species and physical-chemical conditions) that make it significantly different from its surroundings. Thus, a forest is an ecosystem, but a lagoon or a puddle of water in that forest, are themselves ecosystems too. A city is also an ecosystem: a man-made ecosystem.

The limits of many ecosystems are difficult to define. For instance, a lagoon in a forest can be fed with water from an aquifer that extends throughout an area much bigger than the forest itself, and that also feeds with water the farm-land that surrounds the forest. This way, the lagoon, the forest and the farm-land are connected. Ecosystems are not isolated in Nature: there are connections and transitional areas between them.

When an ecologist tries to describe an ecosystem, he/she has to describe the following sorts of facts:

• The set of living species in it, that is, the biocenosis. For instance, in a forest there could be pines, oaks, mice, doves, butterflies, kites, ants…
• The set of physical and chemical conditions, that is, the biotope. In a forest: the average temperature in the coldest month, the average temperature in the hottest month, the rainfall rate, the kind of salts that exist in the soil…
• The relationships between the living species: the pines holding the nests that doves make, the kites eating the mice, the mushrooms feeding on the fallen leaves…
• The relationships between the physical-chemical conditions: how temperatures alter the soil's humidity through evaporation, how wind wears away soil particles…
• The relationships between the physical-chemical conditions and the living species: the way in which the animal activity is affected by the day-night cycle, the way in which temperature affects the loss of water vapour by plants through evapotranspiration, the way plants retain the soil particles with their roots and prevent them from being worn away by the wind, the way in which the decomposition of fallen leaves or dead animals adds new salts and minerals to the soil…

The Ecosphere

Every ecosystem can have other ecosystems inside of it. The Earth itself is one ecosystem and comprises all the other ecosystems. The Ecosphere is the name we give to our planet when we think of it as an ecosystem, and it has a biotope and a biocenosis. The biotope of the Ecosphere is made up by the Geosphere, the Hydrosphere and the Atmosphere. The biocenosis of the Ecosphere is called Biosphere, and comprises all the living beings of the Earth.

The Biomes

If we wanted to split the Ecosphere in the biggest possible ecosystems, we would split it into the biomes. A biome is a large ecosystem made up of all the ecosystems belonging to the same climatic zone. There are, for instance, the tropical rainforest biome, the mediterranean forest biome and the desert biome.

Living beings are formed by clusters of matter increasingly bigger and complex. These clusters of matter are classified in levels: every new level is more complex than the previous one. The simplest organisms (bacteria, protozoa, unicellular algae and yeasts) only reach the cellular level, but the more sofisticated ones also have tissues, organs and systems of organs.

	Examples
Bioelements	C, H, O, N, P, S
Biomolecules	Carbohydrates, proteins, lipids, vitamins, nucleic acids, water, mineral salts
Cells	Sperm cell, palisade cell, muscular cell, neurone…
Tissues	Muscle tissue, nerve tissue, blood…
Organs	Brachial biceps, heart, brain, leaf, root…
Systems of organs	Digestive system, nervous system, circulatory system, skeleton…
Apparatus	Motor apparatus
Multicellular organism	A person, a cat, a fungus, a black poplar…

The bioelements are most abundant chemical elements in a livig being, which are not much the same ones that you can find in a rock or in the air. The top six are C, H, O, N, P, S, and they're called primary bioelements.

The molecules that can be found in all living beings, from the simplest bacterium to the most complex animal, are called biomolecules, the molecules of Life. There are two types:

• Organic: Carbohydrates, proteins, lipids, vitamins and nucleic acids. They all have an inner skeleton built mainly with carbon atoms, which allows for a really large size. The organic biomolecules can only be produced by living beings, if we discard artificial synthesis. This is, they can not be produced by geological or atmospheric processes, for instance.
• Inorganic: Water and mineral salts. They don't have an inner skeleton of carbon atoms and can be produced in non-biological processes.

All living beings are made up of complex structures called cells. Cells are made up of billions of biomolecules working together. Viruses are not regarded as living beings because they are not made up of cells.

Living beings can be (a) unicellular: made up of just one cell (bacteria, protozoa, many algae, yeasts); and (b) multicellular: made up of more than one cell: (some algae, most fungi, plants and animals).

All cells are able to perform the three vital functions: (a) they reproduce, quite usually by mitosis, a process that yields two daughter cells with almost identical genetic material; (b) they interact with their environment, giving responses to specific stimuli, as when a white blood cell is able to detect and destroy a bacterium; and (c) they feed, meaning that they are able to exchange matter and energy with their environment, as when a human cell takes oxygen from the blood and releases carbon dioxide.

All the cells have (a) a cell membrane, which is the cellular envelope, (b) a cytoplasm with organelles, which are specialized cell compartments where specific functions are fulfilled, and (c) genetic material, that carries the instructions that allow both the cellular work and its self-construction.

There are two main kinds of cells: (a) the cells of the bacteria and archaea have no real nucleus: they are said to be prokaryotic; (b) the cells of all the other living beings (algae, protozoa, fungi, plants and animals) have their genetic material separated from the cytoplasm by a nuclear membrane: they are said to be eukaryotic.

The cells of the plants can be easily distinguished from those of the animals because (a) they have a semi-rigid cell wall, made of cellulose, surrounding the cell membrane, that usually gives the cell a polyhedral shape; (b) they have one kind of organelles, called chloroplasts, where sunlight energy is used to start building their own organic substances through a chemical process called photosynthesis; (c) they use to have one or a few big vacuoles that contain sap (instead of lots of smaller ones without sap) which normally push the nucleus out to the periphery of the cell; and (d) although they have equivalent structures, they lack centrosomes, the organelles that control the arrangement of the chromosomes during mitosis in an animal cell.

In multicellular beings there may be different types of cells, each type being specialized in an specific function, and having the specific shape that allows them to fulfill that function the best. Each of those types is called a cellular tissue; examples are the vascular tissue (plants) or the blood tissue (animals). One tissue may have several subtypes of cells (e.g. white blood cells and red blood cells). The human body contains over 200 different types of cells.

The four main types of human tissues are the following:

Epithelial tissue	Composed of layers of cells that line organ surfaces such as the surface of the skin or the inner lining of the digestive tract. Serves for protection of organs (as in the skin), secretion of substances (when it forms glands - in the skin, in the digestive tract, etc.), and absorption of substances (as in the intestine).
Muscle tissue	Composed of very long cells (up to several cm) called muscle fibres. They have more than one nucleus, are able to expand and contract (thanks to a dense protein network that takes up most of the cellular space), and so, are specialized in movements. There are three kinds: cardiac muscle (found in the heart), skeletal muscle (attached to bones and under voluntary control) and smooth muscle (not in the heart or attached to bones and under involuntary control, as in the wall of the stomach).
Nerve tissue	Composed of cells with many projections that are specialized in contacting other cells and transmitting messages via electrical signals.
Connective tissues	Usually specialized in holding together different organs or tissues. It is composed of cells usually very separated by an abundant extracellular matrix. The main types are the bone tissue (in bones, with matrix rich in apatite, a mineral rich in P and Ca), the cartilage tissue (in cartilages), the adipose tissue (as in the fatty layer under the skin - the hypodermis), the fibrous connective tissue (in ligaments and tendons), the loose connective tissue (as in the skin's dermis) and the blood.

There are some tasks in a multicellular being that must be achieved by cells of different kinds working together (such as pumping blood throughout the human body). In this case, cells of different tissues gather and make up an organ (epithelial, connective, muscle and adipose cells make up the heart).

Several organs working together in a common general task make up an organ system, e.g., the heart and the blood vessels make up the circulatory system. And when two organ systems work cosely together in a common function are said to constitute an apparatus: the muscular system and the skeleton form the motor apparatus, because both contribute to the function of locomotion in an animal.

Put simple, the human organ systems contribute to the three vital functions as follows:

• Nutrition is fulfilled through:
• The Digestive System, which (a) takes in the food, (b) breaks it down into nutrients and other substances, (c) absorbs the nutrients into the blood, and (d) gets rid of the non assimilable substances in the form of faeces.
• The Circulatory System (a) transports those absorbed nutrients to all the cells of the body and (b) transports waste substances to the kidneys, the sweat-glands and the lungs.
• The Excretory System expells of the waste substances arriving to the kidneys, by producing and releasing urine.
• The Respiratory System (a) takes in oxygen (a nutrient) which is absorbed into the blood and (b) gets rid of the carbon dioxide (a waste substance).
• Reproduction is carried out through the male and female reproductive systems which (a) produce the specialized reproductive cells (sperm and egg cells), (b) allow those reproductive cells to join in pairs, and (c) grow the embryo coming out of a fertilised egg-cell.
• Interaction is fulfilled through:
• The Sensory Organs, which continuously detect bits of information coming from the inside of the body or from the environment.
• The Nervous System, which collects that sensitive information, interprets it, and generates response orders.
• The Endocrine System, which cooperates in conveying those response orders by means of substances, called hormones, that are released by glands and travel through the blood.
• The Skeleton and the Muscle System, which carry out most of those response orders produced in the nervous system.

Sedimentary rocks can be made up of clasts or detritus (rock grains) weathered from other rocks by river waters, wind, coastal sea waters, glaciers, living beings, etc. This kind of sedimentary rocks are called detrital rocks.

They're classified upon the grains that they are made up of. First up, grains may be all the same size, uniform, as in a sandstone, or different sizes, as in a conglomerate. Secondly, grains may be bigger or smaller. The following table shows roughly how rock grains are classified upon their size:

	Diameter
Clay	< 0.004 mm
Silt	< 0.06 mm
Sand	< 2 mm
Gravel	< 6 cm
Cobble	< 25 cm
Boulders	> 25 cm

Before Darwin the general consensus was that species were created independently by some extranatural force, and that they died out without any major change. So, which were the steps needed for a theory of evolution of the biological species to be accepted in the XIX century?

• Accepting the idea that species can change during their life span.
• Accepting the idea that species can change so much that they can even generate new species, either by diversification (cladogenesis) or by a global transformation of one whole species (anagenesis).
• Accepting the idea (that looks like an inevitable result of the previous one) of a common descent, i. e., that all species are related, have a kinship, and they all come from a common ancestor.
• Accepting the idea that the Earth is old enough for these huge changes to have enough time to occur... so slowly that can't even be perceived during a human life span.
• And most notably: knowing of any acceptable mechanism by which these types of changes can occur.

Considering that not so long ago, the Anglican bishop James Ussher had stated that the Earth had been created the night preceding 23 October 4004 BC, the idea of an old Earth was, quite possibly, the necessary precondition for all the others to be even borne in mind by anyone. And this revolutionary idea was just there in the right moment, when Darwin started his five years long journey in the Beagle, and was given the first volume of Charles Lyell's Principles of Geology, which set out the idea of masses of land slowly rising or falling over immense periods of time to finally yield the geological features that can be observed in present time. According to Lyell, the Earth was much older than what was thought by that time, probably even millions of years old (some 4,560 years old, to be more precise, and as we know today).

The concept of a gradual change of biological species, now that Darwin knew that there was enough time for it, arose from the observation of different races of tortoises, finches and others, adapted to the particular environments that the Galápagos islands had to offer. And more particularly, to the different nutritional niches available; while some finches had a beak specialised in cracking hard nuts, others were the perfect weapon for chasing insects, etc. And considering that those islands are some 1,200 km away from the continent, it was clear to Darwin that all those finches (or tortoises) had had to come from one (or a few) small group of ancestors that, departing from the mainland, happened to make their way to the islands. The Galápagos finches had evolved on-site, as it was highly unlikely that all those varieties could have possibly reached the islands from the continent, one by one.

That panorama was clearly speaking about gradual transmutation of those animals by adaptation to different environments (species can change), and with it, Darwin's mechanism of evolutionary change had started to develop.

But was that slow and gradual change powerful enough to produce new species, or were all those differently adapted finches simple varieties of the same species? Friends are to come to the rescue when needed, and so it was with Darwin's mate John Gould, an ornithologist, who announced that the specimens of finches that Darwin brought back to England after his trip belonged, in fact, to three different species. In Darwin's mind, that meant that gradual adaptation to the environment can actually produce new species.

Finally, how do species get to adapt to each one of the many specific environments that Nature provides? How can a population gradually reach its ecological niche, one that allows it to survive and thrive for generations? That was the last and greatest obstacle to overcome, and recent History showed that clearly. Jean-Baptiste de Lamarck had already come up with similar thoughts to Darwin's a couple of decades before the Beagle departed: species evolve, and do it by gradual adaptation to their environment. But Lamarck could never demonstrate that acquired-on-life traits can be passed on to the offspring, and we know nowadays that that just can't happen: the genes of your eggs or sperms will not change because of you dying your hair. And this is how Lamarck passed to History as an unsuccessful attempt to explain the how and the why of the evolutionary process.

But not Darwin. It turns out that among the many different types of individuals that are randomly produced every generation in every species, some happen to be better suited to their environment than others: the former feed better, grow faster, survive longer and as a result... reproduce more than the latter. And as the traits that make them the fittest were inherited, they will also pass those traits to their children. As a final result, those traits will be more present in the next generation than those that conferred a lesser success. This was what Darwin (and Alfred R. Wallace) elucidated, and that's why everyone is now celebrating his 200^th anniversary. Or almost everyone.

Natural Selection is all about a central event: some individuals reproduce more than others. And this has a cause and a result. Step by step:

• There are many different types of individuals in any biological population. We all can see that. And it is due to mutation and recombination. Mutation increases the number of possible alleles for every loci throughout time, and even creates new loci, or gets rid of loci or alleles. But in the long run mutation tends to increase the number of genes (either alleles or loci) for every species. Mutation takes place mostly due to errors during the duplication of DNA prior to cell division. Also, the recombination of loci during meiosis multiplies the number of different possible gametes that any individual can produce.
• Different types of individuals interact differently with their environment. Some of them are stronger, faster, bigger, smarter... and so some of them grow faster and reach fertility sooner, hide better from predators and manage to sort the threats out better, are more efficient absorbing nutrients or chasing the prey, are more convincing when looking for someone of the opposite sex to mate...
• The individuals that have more successful interactions with their environment leave a greater offspring. No way it can be otherwise. If you reach fertility faster, survive longer, and your nutritional efficiency leaves you more free time, chances are that you are going to have a greater reproductive success.
• Many of the traits that make these individuals more successful are inherited. Although in some species (us!) some "rules for success in Life" can be learned, capability for learning is itself an inherited trait. And, although your genes don't determine the weight that you'll have when you are 25, they do determine a certain range or weights you are likely to fall within.
• The alleles of the individuals with a greater reproductive success will be present with a higher frequency in the next generation, and vice versa. As a result, the phenotypic traits coded by the successful alleles will be more present in the next generation, and this noticeable phenotypic change improves the overall fitness of the population gradually, generation after generation.

In short, Natural Selection takes place through the following sequence of events:

1. Variation due to mutation and recombination.
2. Differential fitness.
3. Differential reproduction.
4. Heritability of phenotypic traits.
5. Change in the frequencies of the alleles of a population.
6. Overall phenotypic change in a population.

The formation of new species or speciation is one of the main outcomes of the evolution of Life, the opposite to extinction, and the most visible result of adaptation.

Sexually reproducing species (virtually all that exist, except prokaryotes) are defined as those groups of individuals that can leave fertile offspring by sexual reproduction, and thus share a common gene pool: the genome of species. This is so because sexual reproduction can be considered as a way to produce new combinations of genes (new genotypes) by means of sharing half of the genes of each one of the two partners. Sexual reproduction is a bit like dealing sets of, say, five cards after having shuffled the deck. In the analogy, each card would be a gene, each set of five cards the genotype of a new individual, and the deck of cards is the genome of the species.

The members of the same species also share the same ecological niche, this is, they dwell on the same habitat, they feed the same way, they prefer a similar amount of humidity or sunlight, the are prey to the same predators, etc.

If two individuals of opposite sex that can leave fertile offspring by sexual reproduction belong to the same species, then, speciation must be the process by which two populations, that originally belonged to the same species, split apart and, after a time of adaptation to different environments (or ecological niches), evolve differently to the end that eventually they become unable to leave fertile offspring by sexual reproduction. This end point of speciation is called reproductive isolation.

This process can take place by different ways. Sometimes the appearance a new geographical barrier is the decisive event. That is the case of the Galápagos tortoises, presumably descendants of a common ancestor, that after colonising the different islands of the archipelago, the sea between the islands posed an almost invincible obstacle for them to meet and mate. This way, the different populations settled down in the different islands, couldn't share a common gene pool anymore, and natural selection made them to evolve differently by improving their adaptation to each particular island. Eventually, they changed so much that if individuals of these different populations happened to meet due to the sea currents or the action of men, they were so different that they would not able to leave fertile offspring by sexual reproduction. Those different populations were now different species.

But speciation does not always need huge geographic barriers. In the case of Galápagos finches, different species appeared in the same islands because when this virgin territory was colonised by their ancestors, a variety of ecological niches were at their disposal, and while some of the finches specialised in feeding off insects, other preferred grains, and so forth. They shared a common habitat, but occupied different places in it and performed different ecological roles. The enhancement of that specialisation (the improvement of their adaptation) to each specific ecological niche by means of natural selection did the rest, and, like the tortoises, with time enough took them to be so different that one day they were reproductively isolated.

But how different two individuals of the opposite sex have to be, to be reproductively isolated? The answer is: either...

• So different that the male wouldn't be able to fertilise the female, because...
• They don't attract each other sexually anymore: courting does not work for them;
• Their genitals have become incompatible;
• The sperms are unable to make their way towards the egg, in the case of mammals;
• etc.
• Or so different that even if the male can fertilise the female, they can't leave fertile offspring, because...
• The zygote can't divide by mitosis (consider a sperm with 23 chromosomes and an egg with 24: this makes a total diploid number of 47, which is odd, and mitosis cannot take place with an odd number of chromosomes);
• The embryo degenerates soon;
• The children are weak and die before sexual maturity;
• The offspring is sterile, as in the case of the mules;
• etc.

What are fossils?

Fossils are the petrified remains of the living beings from the past or of their vital traces. They are studied by the science of the Paleontology.

Fossils are commonly found in sediments or sedimentary rocks (limestone, sandstone, mudstone, shale), typically as a result of the burial of the remains of a living being within a layer of sediments. Heavy metamorphism and the extreme temperatures of the magmas (> 700ºC) are likely to destroy any remain or trace of a living being, and so fossils are not found in heavily metamorphosed rocks (schist, gneiss) or igneous rocks (granite, diorite). Only sedimentary rocks that have undergone a gentle metamorphism, such as slates, are likely to contain fossils.

The totality of fossils and their placement in the rocks containing them constitute the fossil record. This placement may be as important as the fossil itself, because it can give a lot of information about the way and the type of ecosystem in which the fossilised organism lived. For instance, if a fossil is found within a conglomerate, which is a rock formed from the sediments deposited by a river, we'll know that it is one of a land organism. Or if an unknown fossil is found in the same rock where we also find fossils of seashells, it will probably be the fossil of a marine living being.

The fossil record is gappy and uneven. It is gappy because fossilisation is a rare event that happens sporadically and irregularly, depending heavily on the environmental conditions in the moment when a living being dies. If a land animal dies in a place that undergoes a landslide a short time after its death, most likely it will be well preserved inside the sediments. But if it dies in a place where strong winds, or heavy river flow or marine currents drag its corpse for a long time before it is left in a quiet place to be buried as sediments are deposited, chances are that the corpse will be destroyed by those currents, and the scavengers, detritivores and decomposers during the long time that it took before it was buried. And it is uneven because not all species have the same chances to leave any kind of fossil remain. As a start, hard structures (bones, teeth, shells) are commonly necessary, because soft tissues decay rapidly when the scavengers and decomposers (chiefly invertebrates, bacteria and fungi) start to predate on them. Organisms such as jellyfish or worms have really low possibilities to leave body fossils; traces such as imprints or burrows in the sediments are amongst their very few chances to leave a sign of their existence.

Types of fossils

• Body fossils are the petrified remains of living beings from the past, and are produced by the substitution of the biomolecules that pertained to the deceased organism by mineral substances that precipitate from the groundwater that circulates through the sediments in which the corpse is buried. This requires a rapid burial of the organism following its death; otherwise, it will be soon destroyed, as explained above. Other times the remains can be destroyed once covered by sediments, but leaving an organism-shaped hole in the rock: this is called an external mould. If this hole is later filled with other minerals, it is called a cast. An internal mould is formed when sediments or minerals fill the internal cavity of an organism, such as the shell of a bivalve or the skull of a vertebrate.
• Trace fossils are the physical remains of the vital activity of living beings from the past, and are produced by their movement (trackways left by trilobites, footprints from hominans), their reproduction (eggs of dinosaurs), their nutrition (coprolites, gastrolites, holes drilled in the shells of the prey), and other living habits (burrows, root cavities, stromatolites...). The oldest physical fossils on Earth fall into this category. They are stromatolites, and the oldest might be the 3.5 by old found in Warrawoona, Australia. Stromatolites are layered rocks generated by communities of microorganisms, usually dominated by cyanobacteria, which produced the precipitation of mineral substances dissolved in the seawater, generating layers of sediments that stacked one on top of another creating a stratified biogenic rock.
• Biochemical fossils are the biochemical remains of the vital activity of living beings from the past. The best examples are carbon-rich rocks (such as the stromatolites) or minerals (such as graphite granules) that have more ¹²C than usual and less ¹³C than normal. As the CO₂ molecules with ¹²C weigh 44 u instead of the 45 u of a molecule of CO₂ with ¹³C, the former move faster (they are called "light CO₂") than the latter, and have more chances to randomly reach the places of a living being capable of capturing them. This way a plant captures light CO₂ in a greater proportion than it is found in the atmosphere, and so the fossilised remains of a plant will contain ¹²C in a greater proportion than it is found in the atmosphere. The oldest fossils on Earth might be 3.8 by old graphite granules found in Isua, Greenland, that contain a greater than normal proportion of ¹²C.