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In previous lessons you have looked at how natural selection, speciation and human activity can change populations of organisms over tens or hundreds of generations. This lesson zooms out to a much larger timescale. Earth's history stretches back 4.5 billion years, and across that enormous span of time life has evolved, diversified, been repeatedly devastated by mass extinctions, and recovered. You will look at how geologists read this history from rocks and fossils, how they divide it into eons, eras, periods and epochs, and how five great mass extinctions have punctuated it. You will then examine the central HL debate of this unit: whether human activity has pushed Earth into a new geological epoch, the Anthropocene, and what evidence is already accumulating in the rock record to support that case.

Deep time and the fossil record

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3.1.15 Earth history extends over a period of 4.5 billion years. Processes that occur over an extended timescale have led to the evolution of life on Earth.

Include the role of fossils in explaining the evolution of life over the geological timescale.

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Earth formed around 4.5 billion years ago. For most of that time the planet had no living organisms at all, and for the majority of the time since life began, that life was microbial. Complex multicellular life is a comparatively recent arrival. Making sense of timescales this large is genuinely difficult, which is why scientists often use analogies to compress them.

Source

The 24-hour Earth clock. If Earth's 4.5 billion-year history is compressed into a single 24-hour day, with midnight marking Earth's formation and the following midnight marking the present:

• The first simple life (bacteria) appears at around 04:00.
• Oxygen-producing organisms transform the atmosphere at around 11:00.
• The first animals appear at around 20:30.
• The dinosaurs flourish from about 22:45 to 23:40.
• Homo sapiens appears in the final 4 seconds before midnight.
• All of recorded human history fits into the last 0.1 seconds.

This is what geologists mean by deep time. The processes that have shaped life on Earth, including natural selection, speciation, extinction and the slow drift of continents, have operated over durations that are almost impossible to grasp from human experience.

1. State the age of the Earth.

2. Using the 24-hour clock analogy, describe when modern humans appear relative to the age of the Earth.

3. Suggest why analogies such as the 24-hour clock are useful when teaching about geological time.

Fossils as evidence of evolution

A fossil is the preserved remains, impression or trace of an organism that lived in the distant past. Fossils form when an organism is buried in sediment, soft parts decay, and hard parts (bones, teeth, shells, woody tissues) are gradually replaced by minerals as the sediment turns to rock. Over millions of years, earth movements can then lift these rocks to the surface, where erosion can expose the fossils.

Fossils are the primary physical evidence for the evolution of life on Earth. They show that:

• Life has changed through time. The organisms preserved in older rocks are clearly different from those in younger rocks, and different from anything alive today.
• Lineages can be traced. Sequences of fossils show how groups of organisms gradually changed, with transitional forms linking earlier and later species.
• Environments have changed. Marine fossils in rocks now high in mountain ranges show that those rocks were once on the sea floor, which also tells us that continents and sea levels have shifted over time.

Fossilisation is rare and biased. Most organisms never become fossils. The conditions required (rapid burial in sediment, lack of scavenging, low oxygen, stable pressure over millions of years) are unusual. The fossil record is therefore heavily biased towards:

• Organisms with hard parts (bones, teeth, shells, wood), not soft-bodied organisms.
• Organisms living in sedimentary environments, especially shallow seas, lakes and river deltas.
• Organisms that were abundant and widespread, rather than rare or localised.

This bias matters: the fossil record we can read is only a fragment of the life that has actually existed.

1. Define fossil.

2. Outline three conditions that favour fossilisation.

3. Explain why the fossil record under-represents soft-bodied organisms such as jellyfish.

Marine fossils above Ferney-Voltaire