Evidence from fossils

What the fossil record shows

In Year 8 you learnt how fossils form and how rocks are dated using superposition, index fossils and radioactive decay. Put together, millions of dated fossils from every continent give the fossil record: a history of life written in rock. It provides several kinds of evidence for evolution:

• Life has changed over time. The oldest rocks with signs of life contain only microbes. Animals appear much later, and groups appear in a consistent order: fish before amphibians, amphibians before reptiles, reptiles before mammals and birds, and flowering plants late. Fossils are never found out of this order, anywhere in the world.
• Most species that ever lived are extinct. Five mass extinctions, and many smaller ones, cleared the way for new groups to radiate.
• Transitional fossils show features of an older group and a newer one, exactly where evolution predicts they should be.
• Fossils match living species in the same region. Fossil marsupials are found in Australia, fossil armadillo relatives in South America, just as Darwin noticed.

When the biologist J.B.S. Haldane was asked what evidence would disprove evolution, he reportedly answered: "fossil rabbits in the Precambrian." In more than 150 years of searching, no fossil has ever been found in a rock layer that contradicts the order of evolution.

The geological time scale today

Some textbooks still use the names Tertiary and Quaternary for the last 66 million years, with the Quaternary starting 1.8 million years ago. The International Commission on Stratigraphy has updated the time scale: the old Tertiary is now split into the Paleogene (66–23 million years ago) and the Neogene (23–2.58 million years ago), and the Quaternary, the period of ice ages that we live in, begins 2.58 million years ago. The start of the Cambrian, when animals with shells appear, is now dated to about 539 million years ago, and the asteroid impact that ended the age of non-bird dinosaurs to 66 million years ago. Dates like these are refined as dating methods improve; it is the order of events that the fossil record fixes so firmly.

Transitional fossils

A transitional fossil has a mix of features of an ancestral group and a descendant group. It is not usually the direct ancestor itself, but a close relative of it, showing what the in-between stages looked like.

From water to land

Tiktaalik (about 375 million years ago) had scales, fins with fin rays and gills, like a fish. But it also had lungs, a flat head with eyes on top like a crocodile, a neck that let it turn its head without moving its body, strong ribs, and fins containing a shoulder, elbow and wrist. It could probably prop itself up in shallow water. Slightly younger animals such as Acanthostega had true limbs with digits but still had gills.

From dinosaurs to birds

Archaeopteryx, found in Germany in 1861 just two years after Darwin's book, lived about 150 million years ago. It had feathers and wings like a bird, but also teeth, clawed fingers and a long bony tail like a small meat-eating dinosaur. Since 1996, fossil beds in China have revealed dozens of feathered dinosaurs, showing that feathers evolved in dinosaurs long before flight. Birds are living dinosaurs.

From land to sea

Whales are mammals: they breathe air, feed their young milk and have hair as embryos. Since the 1980s, fossils from Pakistan, India and Egypt have shown how they evolved from land mammals related to hippos. The early forms are recognised as whales by a thickened ear bone found only in whales.

Evidence from Australian fossils

Australia's long isolation makes its fossils especially revealing:

• Lightning Ridge, NSW, on Yuwaalaraay and Gamilaraay Country, has fossils turned to opal. In 1985 an opalised jaw about 100 million years old was described as Steropodon, an early monotreme: evidence that the ancestors of platypuses and echidnas were already here when dinosaurs were alive.
• Riversleigh, Queensland, on Waanyi Country, preserves about 25 million years of mammal evolution in limestone, including toothed platypuses, ancient koalas, marsupial lions and early thylacines. The sequence shows Australian rainforest animals changing as the continent dried.
• Megafauna. Until about 40 000 years ago Australia had giant animals, including Diprotodon (a wombat relative weighing up to about 2.7 tonnes), the marsupial lion Thylacoleo, the giant bird Genyornis and short-faced kangaroos. Why they died out is still debated: a drying climate, hunting and changed fire patterns after people arrived, or a combination. First Nations people lived alongside megafauna for thousands of years.
• Rock art as evidence. Paintings in Arnhem Land show thylacines, which disappeared from mainland Australia about 3200 years ago. The art records animals that the fossil record also shows were once widespread.

Living fossils, and why there are gaps

Some living species look very like their fossil relatives. The coelacanth, a lobe-finned fish known only from fossils, was found alive off South Africa in 1938. The Queensland lungfish looks almost the same as fossils about 100 million years old, and the Wollemi pine (Year 7) belongs to a family known from fossils over 90 million years old. These are sometimes called living fossils, but the name is misleading: their DNA has kept changing, and they have simply kept a successful body plan in a stable environment, as stabilising selection predicts.

Why is the record incomplete? Fossilisation is rare, soft-bodied animals rarely fossilise, most rock containing fossils has been eroded, melted or is buried out of reach, and only a tiny fraction of fossil sites have been searched. Gaps are expected. What matters is that every new fossil found fits into the pattern instead of contradicting it, and that predictions like Shubin's succeed.