- Fossils: types, occurrence, significance.
- Historical ideas on the age of the Earth. Principles and procedures that enable to reconstruct the History of the Earth. Relative and absolute dating techniques.
- The geologic time. Main geochronologic units.
- Main geological and biological events along the history of the Earth.
- The evolution of the human lineage.
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.
Main Geochronologic Units
|Eon||Era||Period||Start date (m.y.)|
|Geological events||Time (m.y.)||Biological events|
|· The Big Bang: the origin of time, space, matter and energy, all formed from one single point. The Universe is expanding ever since.||13,700|
|· The Sun, formed from a giant cloud of gas and dust (a nebula), ignites and becomes a young star.|
· The nebula starts taking a flat shape and forms the protoplanetary disk or accretion disc that revolves around the young Sun.
|· The Earth and all rocky matter in the current Solar System start forming by accretion: the accumulation of matter in increasingly bigger nuclei due to the pull of gravity.|
· The Earth's atmosphere initially lacks oxygen.
|· Theia, a planet of the size of Mars, collides with the Earth, which causes a massive ejection of matter into orbit around the Earth, which will finally coalesce to form the Moon.||4,530|
|· Zircons found in Australia are the oldest known minerals.||4,400|
|· The surface of the Earth cools enough for the crust to solidify and the first continents ("shells") form. The atmosphere and the oceans form.||4,100|
|· The Acasta gneisses, in Canada, are the oldest known rocks.||4,030|
|· The inner planets receive the continuous impact of meteors, which probably boiled the oceans away and killed off any form of Life that could have developed.||≤ 3,850|
|3,800||· First possible fossils: chemical imprints of Life in graphite granules found in the oldest known rocks with a sedimentary origin, in Greenland.|
· The first living beings were similar to prokaryotes, and obtained the carbon from CO2 and the energy from inorganic substances such as H2S, that could have been obtained from the thermal vents that are found in the undersea tectonic boundaries.
|3,430||· First possible physical fossils: possible biogenic stromatolites found in Australia. These are layered rocks created by a multispecific community of microorganisms dominated by cyanobacteria.|
|· The concentration of oxygen starts to rise in the Hydrosphere and the Atmosphere, which is the most critical ecological change in the History of Earth, killing off most prokaryotes (which were anaerobic) and paving the road for the evolution of all the aerobic forms of life, including plants and animals.||2,400|
|2,100||· Eukaryotic cells appear, probably derived from prokaryotes engulfing others via phagocytosis.|
|· Supercontinent Columbia.||1,700|
|1,200||· Sexual reproduction appears, increasing the rate of evolutionary change.|
· First multicellular organisms appear as colonies of cells with some kind of division of labour.
|· Supercontinent Rodinia.||1,000|
|· Ice age: Snowball Earth.||750|
|· Supercontinent Pannotia.||580||· Ediacara biota: first complex multicellular organisms.|
|540||· Cambrian explosion: most modern types of animals appear, including trilobites, ancestors of modern arthropods.|
|500||· First vertebrates (fish).|
· First land plants.
· First land fungi.
· First land arthropods.
|380||· Amphibians, first four-limbed tetrapods, evolved from fish, start colonising the continents.|
· Fern forests start to dominate the land.
|· Supercontinent Pangaea starts forming.||300|
|250||· A massive extinction at the end of the Permian eliminates 95% of living species.|
|230||· Dinosaurs appear.|
· Seed plant (gymnosperms) forests start to dominate the land.
|· Supercontinent Pangaea starts breaking up.||180|
|130||· Rise of angiosperms (flowering plants).|
|66||· A massive extinction at the end of the Cretaceous, possibly caused by a 10 km across meteorite that left the crater of Chicxulub, in Mexico, eliminates about half of all animal species, including all dinosaurs (except the ancestors of modern birds) and ammonites.|
· Mammals will take advantage of this event and will diversify rapidly, occupy most ecological niches left by dinosaurs, and become the dominant vertebrates on land.
|6-7||· Hominans (biped primates) appear in Africa, maybe with Sahelanthropus tchadensis (-7my) or Orrorin tugenensis (-6my).|
|2.5||· Humans (Homo habilis) appear in Africa.|
|0.2||· Modern humans (Homo sapiens) appear in Africa.|
|0||· With a human population approaching 7 billion, the impact of humanity is felt in all corners of the globe. Overfishing, anthropogenic climate change, industrialisation, intensive agriculture, clearance of rain forests and other activities contribute to a dramatically rising extinction rate.|
Paleontology and Dating Techniques
Questions: Earth's Timeline
- Why did the first oceans and continents form at about the same time?
- All or almost all rocks known from the Archean eon are metamorphic. Why?
- According to the biochemical fossils of Isua (Greenland), it is said that Life appeared on Earth in the very first moment that it was possible. Why?
- The oxygen pollution was necessary for Life to colonize the continents. Why?
- Which eukaryotic organelles are thought to have evolved from bacteria engulfed by the primitive eukaryotic ancestor?
- Why did sexual reproduction appear so late in the History of Life on Earth?
- Why does sexual reproduction increase the rate of evolutionary change?
- Considering the huge amount of fossils that these organisms left behind, the Paleozoic is known as the era of the ...
- From which geological time comes most of the coal that we consume nowadays?
- Considering the way these animals dominated the land ecosystems, the Mesozoic is known as the era of the ...
- The rise of angiosperms in the late Mesozoic must have boosted the evolution of ...
- How can a meteorite cause such a massive extinction as that of the end of the Mesozoic?
- Some scientists say that dinosaurs have not died off, and that they are still among us. Why?
- Mammals had existed throughout all the Mesozoic, but why didn't they flourish until the Cenozoic?
- Why are fossils unlikely to be found in granite?
- Why are fossils unlikely to be found in gneiss?
- Why can fossils be found in sandstone?
- Why can fossils be found in slate?
- An unknown fossil is found in a rock with symmetrical ripple marks, such as those left in the sand by the sea waves. What sort of information does this provide about the life habits of the organism that left the fossil?
- Can you tell any other way by which a sedimentary rock can provide useful information about the fossils found in them?
- What is the difference between an internal mould and a cast?
- What sort of fossils can be expected from a poppy? And from an oak tree? And from a grasshopper?
- What kind of trace fossils can a tiger leave? And a trilobite?
- Why do you have a greater proportion of 12C than the atmosphere?
Reading: How the Discovery of Geologic Time Changed our View of the WorldSource
Imagine trying to understand history without any dates. You know, for example, that the First World War came before the Second World War, but how long before? Was it tens, hundreds or even thousands of years before? In certain situations, before radiometric dating, there was no way of knowing.
By the end of the 19th century, many geologists still believed the age of the Earth to be a few thousand years old, as indicated by the Bible, while others considered it to be around 100 million years old, in line with calculations made by Lord Kelvin, the most prestigious physicist of his day.
Dr Cherry Lewis, University of Bristol, UK, said: "The age of the Earth was hugely important for people like Darwin who needed enormous amounts of time in which evolution could occur. As Thomas Huxley, Darwin's chief advocate said: 'Biology takes its time from Geology'."
In 1898 Marie Curie discovered the phenomenon of radioactivity and by 1904 Ernest Rutherford, a physicist working in Britain, realised that the process of radioactive decay could be harnessed to date rocks.
It was against this background of dramatic and exciting scientific discoveries that a young Arthur Holmes (1890-1964) completed his schooling and won a scholarship to study physics at the Royal College of Science in London. There he developed the technique of dating rocks using the uranium-lead method and from the age of his oldest rock discovered that the Earth was at least 1.6 billion years old (1,600 million).
But geologists were not as happy with the new results as, perhaps, they should have been. As Holmes, writing in Nature in 1913, put it: "the geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the Earth's crust, is still more embarrassed with the superabundance with which he is now confronted." It continued to be hotly debated for decades.
Cherry Lewis commented, "In the 1920s, as the age of the Earth crept up towards 3 billion years, this took it beyond the age of the Universe, then calculated to be only 1.8 billion years old. It was not until the 1950s that the age of the Universe was finally revised and put safely beyond the age of the Earth, which had at last reached its true age of 4.56 billion years. Physicists suddenly gained a new respect for geologists!"
Answer the following questions:
- How do the radiometric dating techniques work?
- Why were the radiometric dating techniques so important for Biology after Darwin's time?
Reading: Brief History of PaleoanthropologySource: Popsci.com
In the beginning...
Today, it's a widely accepted fact that humans originated in Africa. But less than a century ago, anthropologists assumed that Eurasia was the birthplace of humanity. And scientists held onto that mistaken belief until one man took a stand that rewrote history.
In 1923, Raymond Dart arrived at the University of the Witwatersrand in Johannesburg to take a post as the head of its anatomy department. The 30-year-old Australian physician, an expert in neuroanatomy, was disappointed to learn that the university did not own a reference collection of bones and fossils. He set out to amass one, offering his students a prize for the most interesting bones they could find. His lone female student, Josephine Salmons, soon presented him with a South African fossil that would lead to the discovery of a lifetime. The fossil, a baboon cranium, sparked Dart's interest, since only two primate fossils had been found in sub-Saharan Africa until then. Salmons had found the fossil at the home of the director of the Northern Lime Company at Taung, a mining site in South Africa. Dart asked the manager of the site to alert him to any fossils that his miners unearthed in the future.
The Child of Taung
One Saturday in 1924, two boxes of rocks from Taung were deposited at Dart's door. In the second box, he came across an exciting find: an endocast, the fossilised imprint of an animal brain. To his astonishment, the endocast showed a brain larger than that of a chimpanzee but smaller than those of known human ancestors.
Digging through the box, Dart located the matching limestone rock that he knew might contain the face to match the brain. His thoughts turned to Charles Darwin, who, in his 1871 book The Descent of Man, predicted that humans' earliest apelike ancestors would be discovered in Africa because our closest ape cousins - chimpanzees and gorillas - lived there. Darwin's prediction had been generally discredited. After all, only two ancestral human fossils had ever been found in Africa and both were relatively recent, closer to modern humans than to apes. Dart wondered whether he had stumbled upon proof of Darwin's controversial theory.
For the next 73 days, Dart scraped away at the matrix, the limestone surrounding the fossilised face. Finding his hammer and chisel too clumsy, Dart turned to his wife's knitting needles, which he had sharpened to a point. His efforts were rewarded when the rock finally parted to reveal the face of an apelike child with a full set of baby teeth and molars beginning to appear.
Despite the primitive face, the child's skull, teeth and jaw clearly resembled those of humans, and the position of the opening at the back of the skull, where the spinal cord meets the brain, indicated that it walked upright, known as bipedalism. Here, Dart realized, was an early human ancestor. In a 1925 article in the journal Nature, Dark introduced the new species, which he named Australopithecus africanus, meaning "southern ape from Africa". The fossil became known as the Taung child.
Dart's discovery set off a firestorm. The leading European scientists, who preferred to believe that humans originated closer to home, were skeptical of his find. An abundance of physical evidence, including fossils and cave paintings, seemed to point to Eurasia as the cradle of humankind, and a British fossil known as Piltdown man was the most convincing evidence that Dart was incorrect. Charles Dawson, an English soldier and amateur antiquarian, discovered the bones in a gravel pit in Piltdown Common in Sussex between 1910 and 1912. Not only was Piltdown man conveniently European, but it looked like what scientists expected human ancestors to look like. It had a simian jaw and teeth but a modern-human-size brain. At the time, scientists assumed that our large brain evolved before other human traits. Piltdown man was, therefore, considered irreconcilable with the Taung child, with its humanlike jaw and teeth but small brain.
In Europe, scientists and the press widely dismissed the Taung child and ridiculed Dart. Most paleontologists believed the fossil was of a young ape, most likely a chimpanzee. Dart traveled to England in 1931 to show the fossil at scientific conferences, but he did not gain many supporters. He did not touch the fossil again for years.
Dart was not the first researcher whose fossil was unfairly rejected by European scientists. In 1893, Dutch physician Eugene Dubois announced the discovery of a set of well-preserved fossils from the Indonesian island of Java. Dubois felt that the three bones - a skullcap, molar and femur - belonged to a human ancestor that walked upright. He called the species Pithecanthropus erectus, or upright ape-man, and the fossil became known as Java man.
When he returned to Europe, Dubois was surprised to be met with resistance. Like the Taung child, Java man's skullcap indicated a relatively small brain. The human brain, scientists said, must have reached modern proportions before the creature could have been capable of walking upright. Dubois's colleagues immediately dismissed the fossil as a large gibbon or a deformed modern human.
In 1940, Dubois died without receiving the credit he was due. Java man has since been reclassified as Homo erectus, the enormously successful species that thrived for 1.5 million years starting around 1.7 million years ago and that may have directly preceded Homo sapiens, or modern humans. As the first H. erectus specimen ever found, the 900,000-year-old Java man is among the most important human-lineage, or hominan, fossils in the world.
New African discoveries
Fortunately for Raymond Dart, thanks to some important allies, recognition came within his lifetime. Although European scientists rejected his findings, his article in Nature inspired a colleague, the Scottish scientist Robert Broom, to prove that Dart's fossil was indeed an early human and that our earliest ancestors hailed from Africa. At the age of 70, Broom, a respected paleontologist and curator of vertebrate fossils at the Transvaal Museum in Pretoria, set out to find an adult Australopithecus in Africa - something to silence Dart's critics once and for all.
Broom found such evidence in 1936, when he began acquiring fossils from caves at Sterkfontein, a site just south of Johannesburg. His early finds included the skull of an adult australopithecine. After World War II, Broom found the limb bones of a different hominan species, Paranthropus robustus, at a nearby site. With the two fossils in hand, Broom was able to confirm that these early ancestors walked upright.
These finds infected Dart's students with enthusiasm, and one of them convinced him to go fossil-hunting in the 1940s. In the caves at Makapansgat, 150 miles north of Sterkfontein, Dart and his colleagues found several australopithecines. But Dart wasn't truly vindicated until 1947, when Broom found the fantastically well-preserved skull of an adult female A. africanus at Sterkfontein. The evidence could no longer be denied: The Taung child was a hominan, not an ape. That same year, the British Association for the Advancement of Science passed a resolution stating that Broom's discoveries provided "a vindication of the general view put forward by Professor Raymond Dart in his report of the first Australopithecus skull found in 1924."
A theory takes hold
By 1953, Piltdown man had become a clear anomaly in the growing hominan fossil record. That year, scientists at Oxford and the British Museum revealed that the English fossil had, in fact, been a hoax. The culprit - whose identity remains a mystery - pieced together the fossil from a 600-year-old human cranium, an orangutan jaw and teeth, and perhaps a chimp tooth. The bones had been chemically stained and the teeth worn down to mimic human usage. With Piltdown man out of the way, the African-origin theory gained widespread acceptance.
The final confirmation came in 1959, when British archaeologist Mary Leakey made an unexpected discovery in East Africa's Rift Valley. Out walking her dogs one morning in Tanzania's Olduvai Gorge, Leakey noticed a bone sticking out of the sand. When she brushed the dirt off, she was staring into the dark eye sockets of a near-complete skull of a previously unknown species, one that her husband, archaeologist Louis Leakey, later named Zinjanthropus boisei. The fossil, with its huge cheekbones and sharply crested skull, was an amazing find. And like the Taung child, it had a brain smaller than that of modern humans but larger than modern apes.
In 1961, geologists at the University of California at Berkeley used a new technique called potassium-argon dating to accurately place the species, whose scientific name had been changed to Paranthropus boisei, in history. According to this groundbreaking method, which involves dating the rocks surrounding the specimen, the skull was 1.75 million years old - four times as old as previously thought and 750,000 years older than the accepted time frame for all of human history.
Nutcracker man, as the skull was known, was the conclusive fossil evidence that the earliest human ancestors lived in Africa, despite the fact that they didn't look like what scientists expected. Researchers streamed to East Africa in a fossil rush aimed at finding our earliest ancestors. Over the next few decades, a series of spectacular discoveries in East Africa pushed human origins deeper into the past.
In 1974, researchers in the Afar region of Ethiopia discovered "Lucy", an astonishingly complete three-million-year-old A. africanus skeleton. Lucy could clearly walk upright and was considered our oldest ancestor for a time. More recently, fossil finds have pushed the date of our earliest ancestor back further, to between five million and six million years ago, and in 2002, French paleontologist Michel Brunet announced the discovery of the oldest species yet, the six-to-seven-million-year-old Sahelanthropus tchadensis. Brunet found Toumaï, as the oldest hominan fossil ever found has come to be known, in the Djurab Desert of northern Chad.
In the 1960s, when potassium-argon dating was developed, paleontology began to go high-tech. For the first time, fossils older than 50,000 years could be dated. Genetic technology also came into play that decade, and today it is revolutionizing the study of human origins.
Vincent Sarich and Allan Wilson of the University of California at Berkeley were pioneers in the application of genetics to paleoanthropology in the 1960s. Their comparisons of chimpanzee and human DNA showed that the chimp lineage split from our common ancestor five million years ago. At the time, the findings caused an uproar. Anthropologists, who believed that humans and chimps diverged 15 million years ago, rejected the theory.
Recent genetic studies, however, support Sarich and Wilson's early work. The complete human genome was published in 2004, and the chimp genome followed in 2005. That year, scientists at Arizona State University and Pennsylvania State University compared modern human mitochondrial, or maternal, DNA with chimpanzee, macaque and mouse DNA to determine the point at which each lineage diverged from our common ancestor. "Though we will never know the exact date of the split, we can estimate that date using differences in their DNA", explains Blair Hedges, an evolutionary biologist at Penn State. These differences, or mutations, are assumed to occur at a constant rate, which can be used to estimate how much time has passed since lineages diverged. This method, called the molecular clock, indicated that the human and chimp lineages split five million to seven million years ago, although more fossil-based research is needed to confirm that idea.
- Foramen magnum.
- Bipedalism / Bipedality.
Answer the following questions:
- Which was the presumed cradle of Mankind by the beginning of the XX century?
- What discovery lead finally to the dismissal of that theory?
- What species does the Child of Taung belonged to?
- What is an endocast?
- What type of human evolution was suggested by the Piltdown Man?
- And what type of human evolution was suggested by the Child of Taung?
- What do the huge cheekbones and sharply crested skull of Paranthropus boisei tell about its life habits?
- How does the genetic clock technique work?
- What does the genetic clock technique say about the split date between human and chimpanzee lineages?
- Which is the oldest hominan known to date? How old is it?