The Origin and Development of Life
Where Did Life Begin?
The oldest confirmed evidence of life on Earth comes from stromatolites — layered mats produced by microbial communities — found in rocks approximately 3,500 million years old (3,500 Ma). This tells us that life had already appeared within the first billion years of Earth’s history. How it began is one of the most actively researched questions in science, and two environments are considered the most plausible settings: hydrothermal pools on land, and deep-ocean black smokers on the seafloor.
Both settings share key features: they concentrate chemical compounds, supply energy (heat and chemical gradients), and could have provided the conditions for simple organic molecules to form and combine into more complex structures. The deep-ocean black smoker hypothesis has gained support because early Earth’s surface was subjected to intense ultraviolet radiation (there was no ozone layer yet), so deep-sea environments would have been more protected. Neither hypothesis has been conclusively proven, and the origin of life remains an open scientific question.
The oldest evidence of life on Earth is approximately 3,500 Ma old — preserved as stromatolites (layered microbial mats) in ancient rocks. This places the origin of life within the first billion years of Earth’s history, during the Archean eon.
From Single Cells to Complex Life
For the first two billion years after life appeared, it remained microscopic and single-celled. The first step up in complexity came with the appearance of cells containing a nucleus (eukaryotes) around 1,800 Ma, and the first multicellular organisms appeared around 600 Ma. The Phanerozoic eon, which begins at the Cambrian (541 Ma), is defined by the appearance of hard-bodied animals whose remains preserve well as fossils.
The Cambrian Explosion — a geologically rapid diversification of animal body plans — is spectacularly documented in the Burgess Shale of British Columbia, Canada (approximately 508 Ma). The Burgess Shale preserves soft-bodied organisms that would normally leave no trace, giving a rare window into the full range of Cambrian life. From this Cambrian foundation, the major animal lineages diversified through the Palaeozoic era (541–252 Ma): fish appeared in the Ordovician, amphibians colonised the land in the Devonian, reptiles first appeared in the Carboniferous and dominated the land by the Permian, and mammals and birds diverged from reptile lineages in the Mesozoic era.
The sequence of first appearances in the fossil record follows evolutionary complexity: fish (Ordovician), amphibians (Devonian), reptiles (Carboniferous), mammals (Triassic), birds (Jurassic). Mammals and birds both evolved from reptile lineages, not from each other. This sequence is the basis for cladograms in your exam.
Landmark Fossil Finds
Archaeopteryx, discovered in the Solnhofen Limestone of Germany in 1861, is one of the most famous fossils in science. Aged at approximately 150 Ma (Jurassic), it shows a combination of bird-like features (flight feathers) and reptilian features (teeth, clawed fingers, a long bony tail). It is considered a key transitional fossil showing the link between non-avian dinosaurs and modern birds.
Lucy (Australopithecus afarensis) is a partial skeleton discovered in Ethiopia in 1974, aged approximately 3.2 million years old. Her fossilised bones show features of both upright walking (the shape of her knee and pelvis) and tree-climbing (curved finger bones). Lucy is a crucial fossil in understanding the evolution of hominids — the group that includes modern humans and their immediate ancestors.
Mass Extinctions
The history of life is not one of steady progressive improvement — it is punctuated by catastrophic mass extinctions. There have been five major mass extinctions recognised in the Phanerozoic fossil record. The most severe was the end-Permian extinction (approximately 252 Ma), which wiped out an estimated 90 to 96% of marine species. The most famous is the Cretaceous-Palaeogene (K-Pg) extinction at approximately 66 Ma, which ended the non-avian dinosaurs and many other groups, clearing the ecological space for mammals to diversify.
The K-Pg extinction is supported by a global layer of iridium-rich clay found at the Cretaceous–Palaeogene boundary in rocks worldwide. Iridium is rare in Earth’s crust but abundant in meteorites, leading to the hypothesis (developed by Luis and Walter Alvarez in 1980) that a large asteroid impact contributed to the extinction event. Evidence for the impact includes the Chicxulub crater in Mexico (approximately 200 km in diameter), dated to approximately 66 Ma. Volcanic activity from the Deccan Traps in India at the same time also contributed to environmental disruption.
A cladogram is a branching diagram that shows evolutionary relationships. Each branch point (node) represents a common ancestor shared by the groups on either side of the split. In geology and palaeontology, cladograms are built using both fossil evidence (which body plan appeared first, which features are shared) and, for more recent organisms, molecular evidence (DNA comparisons).
A simple cladogram for vertebrates would place fish as the earliest branch, with amphibians diverging next (sharing the bony limb inherited from fish), then reptiles (adding the amniotic egg), then birds and mammals branching from reptile lineages. In the exam, you may be asked to read a cladogram — identify which group is most closely related to which, or use the cladogram to work out the order in which features evolved. The node is the key: groups that branch from the same node share a common ancestor and are more closely related than groups that share only a deeper node.
The fossil record is a biased sample of past life. Organisms with hard parts (shells, bones, teeth) are far more likely to be preserved than soft-bodied organisms, which is why the Burgess Shale is so exceptional — it shows us what we normally miss. Preservation requires rapid burial in sediment, which is more likely in marine and lake environments than on land. As a result, marine invertebrates are heavily over-represented in the fossil record compared to terrestrial vertebrates.
Even well-preserved organisms may never be found — they may be eroded away, buried too deeply, or simply never encountered by geologists. This means the fossil record always underestimates biodiversity and can never give a complete picture of ancient life. When a new fossil is found that extends the known range of a group earlier in time, it does not contradict evolution — it fills a gap that was always expected to exist. Be ready to explain why the absence of a fossil in one rock unit does not prove that organism did not exist at that time.