10.2 Representations of life's history

Geological timescale (ESGCC)

The geological timescale is a 'calendar' of events in the Earth's history. It shows major geological and climactic events, and how these events affected the emergence and disappearance of species over time. In order to help us make sense of this vast amount of time, scientists divided it into smaller units of time. In descending order, these units are: eons, eras and periods.

• An Eon: is defined as a unit of time equal to a billion years.

• An era: is a division of time within an eon but does not have a fixed number of years. The Mesozoic era for example lasted from 252 million years to 66 million years ago.

• A period: typically refers to a subdivision of an era and its length is determined by a system of dating based on examining fossil evidence belonging to a particular era.

Figure 10.1 below shows one method of representing the geological timescale:

While you will not be expected to remember the names of specific periods, you will be expected to understand the eras and their particular characteristics.

Another visually powerful way of representing Earth's history is via the use of the geological `clock' (Figure 10.2). Human history occupies just 2 million of the \(\text{4 500}\) million year long history of the Earth. On this clock, human existence constitutes less than a minute of the evolutionary history of life on Earth.

Continental Drift (ESGCD)

Continental drift is the breakup and movement of the Earth's continents relative to each other by drifting across the Earth's surface. Since the initial continental drift hypothesis was proposed, the study of plate tectonics has helped us understand why continents move.

Plate tectonics is the study of the folding and faulting of the Earth's crust (lithosphere).

Biogeography is the branch of biology focusing on the geographical distribution of plants and animals. It has been instrumental in developing our understanding of the evidence for continental drift.

Evidence for continental drift

There is considerable evidence for the theory of continental drift that draws upon fossil evidence, plate tectonics theory and studies of glacier sediments. For example:

• Similar plant and animal fossils have been found on different continents' shores, suggesting that these were once joined. For example:
  • The Mesosaurus found in Brazil and South Africa.
  • The Lystrosaurus from rocks of the same age found in South Africa, South America, India, Australia and Antarctica
  • Earthworms found in South America and Africa suggest that these existed in a common habitat on a single continent.
• The complementary shapes of South America and Africa have enabled scientists to propose how these continents pulled apart due to various plate tectonic forces.
• The study of glaciers left over from the ice ages has provided an important line of evidence for continental drift. Glacial sediments from South America, Africa, India, Madagascar, Arabia, India, Antarctica and Australia showed evidence of having once been joined together, suggesting the existence of the supercontinent Gondwana.

Using this evidence, scientists have inferred that in the past the Earth existed as a super-continent known as Pangea until the early Mesozoic era. There were three major phases in the break-up of Pangea:

• First phase: about \(\text{175}\) million years ago, Pangea began to rift, giving rise to the supercontinents Laurasia and Gondwana.
• Second phase: began in the early Cretaceous period (\(\text{150}\)–\(\text{140}\) million years ago) when Gondwana broke and separated into multiple continents: Africa, South America, India, Antarctica and Australia.
• Third phase: occurred in the early Cenozoic era. During this phase, Laurasia split when North America/Greenland split from Eurasia resulting in the expansion of the Atlantic and Indian Oceans.

Natural selection (ESGCF)

• Individuals in a species show a wide range of variation: .e.g. the many different types of dogs you see.
• The variation is caused by genetic differences.
• Individuals that are well-adapted to the environment in which they live are more likely to survive and produce more offspring.
• The genes that allowed the individuals to be successful are passed on to the offspring in the next generation.
• Over many hundreds (and thousands) of generations this results in changes in the physical appearance and habits of organisms.

Fossil formation (ESGCG)

The fossil record is a key source of evidence that helps scientists understand life's history. Fossils are the remains, impressions or traces of animals and plants from the remote past. Fossils usually take the form of a mold or a cast in rock. Generally, a preserved specimen is regarded as a fossil if it is older than approximately \(\text{10 000}\) years (although this is not a strict definition).

Examples of common fossils are skeletons or parts of skeletons, shells or teeth. Sometimes plants or animals can leave imprints that get preserved as fossils.

The study of fossils across geological time, how they are formed, and how ancient organisms have evolved in relation to other phylogenetic groups, is called palaeontology.

How fossils are formed

1. For fossilisation to occur, a plant or animal must first die. Soft tissues decay quite quickly, therefore animals that have hard exoskeletons and woody plants tend to fossilise better than soft-bodied organisms.

   

2. The organism (plant or animal) must be buried beneath mud and or soil shortly after death. Although decay still takes place, the lack of oxygen slows it down. As more and more layers of mud and soil are added, the sediments become compressed.

   

3. Eventually, this compression turns the sediment into rock, which forms a mould around the shape of the original skeleton. Sometimes the original bone or shell softens and dissolves completely, sometimes the bone or shell remains. Water that is rich in dissolved minerals trickles in through the layers of sediment into the mould.

   

4. The mineral-rich water enters the hollow and crystallises to create a natural cast of the original organism. Otherwise, the minerals slowly seep into the skeleton, changing its chemical composition and making it capable of surviving for a long time.

   

5. Over many millions of years, rock that was once buried rises again to the surface and is eroded away, exposing the fossils.

   

Fossilisation is very rare and only happens when a plant or animal dies in exactly the right circumstances. Usually animal corpses are eaten by other animals or decomposed by bacteria before fossilisation can occur. Even hard parts, like bones and shells, are eventually destroyed through erosion and corrosion.

Methods of dating fossils (ESGCH)

• The carbon atom exists as three different isotopes. These are carbon-12 (C-12), carbon-13 (C-13) and carbon-14 (C-14).
• All living organisms maintain an equilibrium of carbon-14 with the atmospheric carbon-14.
• After an organism dies, it no longer incorporates new carbon into its body.
• The carbon-14 present within them undergoes radioactive decay to nitrogen-14, and decreases from the original equilibrium with carbon-12.
• The half-life of carbon-14 is 5730 years. This means that if we start out with say 10 units of carbon-14, in 5730 years' time only 5 units will remain. In another 5730 years' time only 2.5 units will remain and so on.

• Thus, using radiocarbon dating, scientists can determine how much carbon-14 remains within a particular fossil, and thereby infer the age of the fossil.

The isotopes of different elements undergo decay at different rates; some decay much more rapidly than others. This makes radiometric dating a useful tool as scientists can use different elements to date longer and shorter time-scales. Carbon has a relatively short half-life, and therefore it is not of much use when dating fossils that are millions of years old.

Uranium-lead and potassium-argon dating

In order to date older fossils, scientists use uranium-lead dating and potassium-argon dating. The half-life of uranium-235 is 700 million years, and the half-life of potassium-40 is 1.3 billion years. In order to estimate the age of incredibly old fossils, scientists date the age of the igneous (volcanic) rock in which the fossils are buried. This provides them with an estimate of the age of the fossils contained within them.

2. Relative Dating

• Relative dating is the method of determining the order of events from the fossil record.
• By studying the order in which fossils occur in the fossil record, geologists can determine the order of events as they occurred but not when exactly they occurred.
• Fossils found at the lowest layer of rock would be the oldest, as these would have been buried for the longest time, whereas fossils found closer to the surface would be buried more recently and therefore be younger.
• The geological time-scale you studied earlier was almost entirely developed by relative dating methods. It is a useful method of dating when fossil materials lack radioactive isotopes.

Deductive reasoning to understand the past (ESGCJ)

Deductive reasoning combines the use of evidence and theories to make deductions about the past. Therefore scientists use their understanding of continental drift and natural selection theories, together with evidence of climate changes and extinct organisms from the fossil record, to piece together Earth's history.

• Scientists use the fossil record to make conclusions about the history of life through a process of deductive reasoning.
• Deductive reasoning involves combining our understanding of known principles to make conclusions about new evidence that we have uncovered.
• Our knowledge of the history of life is not based on radiometric dating methods alone. Rather, our understanding of the changes in Earth's climate and biogeography allow us to make conclusions about newly discovered fossil evidence.
• For example, from our knowledge of the changes to the Earth's early atmosphere, we know that the formation of the ozone layer blocked off the damaging rays of the sun's UV rays. This led to the growth of plant species which gradually made terrestrial existence possible.

• A transitional fossil is any fossilised remains that is common to an ancestral life form as well as to the group that is derived from it.

• Transitional fossils give us information about how an ancestral species evolved to form the existing species.

• An example of a transitional fossil is the Archaeopteryx. It is thought to belong to the genus of Theropod dinosaur which is closely related to the birds.

The exercise below requires you to understand the similarities between the Archaeopteryx and the modern bird.