Evidence for Evolution 1: The Fossil Record

A fishapod from Nunavut

In 2004, a team of palaeontologists working on Ellesmere Island in Nunavut found the fossil of an animal that lived about 375 million years ago. It had scales, fins and gills like a fish, but also a flat head, a neck and sturdy fin bones with wrist-like joints, like the earliest four-limbed animals. They named it Tiktaalik, an Inuktitut word for a large freshwater fish, chosen with advice from Inuit Elders in Nunavut.

The team did not stumble on it by luck. They predicted where to look: in rocks of the right age (between fish-only rocks and rocks with early four-limbed animals) and the right kind (from ancient streams). Finding a transitional fossil exactly where the theory predicted is powerful evidence for evolution. This lesson explains how the fossil record works.

How fossils form

Most organisms rot or are eaten and leave no trace. Fossils form only in special conditions, most often when an organism is buried quickly in sediment (mud, sand, volcanic ash) where there is little oxygen. Hard parts such as bones, shells, teeth and wood are most likely to be preserved.

Kinds of fossils:

• Mineralized (petrified) fossils: minerals in groundwater slowly fill or replace the original material, turning bone or wood to stone.
• Moulds and casts: an organism dissolves, leaving a hollow mould in the rock; if minerals fill the mould, they form a cast.
• Trace fossils: footprints, burrows, nests and coprolites (fossil dung). They show behaviour.
• Preserved remains: whole organisms trapped in amber (tree resin), tar or permafrost. Woolly mammoths and other Ice Age animals have been found frozen in the permafrost of Siberia and the Yukon.
• Carbon films and soft-bodied fossils: in very fine mud, even soft bodies can leave detailed outlines. The Burgess Shale in Yoho National Park, British Columbia, holds amazingly detailed soft-bodied animals about 508 million years old. It was discovered by the American palaeontologist Charles Walcott in 1909 and is now part of a UNESCO World Heritage Site.

Reading the rock layers

Sedimentary rock forms in layers, with new layers settling on top of older ones. So, in undisturbed rock, the lower the layer, the older it is. This is the law of superposition. It lets us put fossils in relative order (oldest to youngest) even without knowing their exact ages.

When palaeontologists compare layers around the world, they always find the same broad pattern:

• the oldest rocks with life hold only single-celled organisms;
• simple multicellular organisms appear later;
• fish appear before amphibians, which appear before reptiles, which appear before mammals and birds;
• flowering plants appear long after the first land plants.

No one has ever found a rabbit in rocks older than the first fish. That pattern is exactly what descent with modification predicts.

Putting numbers on time: radiometric dating

Many rocks and fossils contain radioactive isotopes: unstable atoms that decay into other atoms at a steady rate. The half-life is the time for half of the radioactive atoms in a sample to decay. Half-lives are not affected by temperature or pressure, so they work like a clock.

• After 1 half-life, 50% of the original isotope remains; after 2 half-lives, 25%; after 3, 12.5%; and so on.
• Carbon-14 has a half-life of about 5730 years. It is used to date the remains of organisms (bone, wood, shells) up to about 50 000 years old.
• Older rocks are dated with isotopes that have much longer half-lives. For example, potassium-40 decays (partly) to argon-40 with a half-life of about 1.25 billion years. Volcanic ash layers above and below a fossil can be dated this way to bracket the fossil's age.

Radiometric dating tells us Earth is about 4.6 billion years old, confirming Hutton and Lyell's deep time.

Transitional fossils

If groups of organisms evolved from earlier groups, we should find fossils that show mixtures of traits, in rocks of the right age. Palaeontologists have found many:

• Tiktaalik (about 375 million years old, Nunavut): fish features plus a neck and limb-like fins. It links fish and the first four-limbed vertebrates (tetrapods).
• Archaeopteryx (about 150 million years old, Germany): feathers and wings like a bird, but teeth, clawed fingers and a long bony tail like small dinosaurs. It links dinosaurs and birds.
• Early whales such as Pakicetus and Ambulocetus (about 50 million years old, from Pakistan): land mammals with ear bones like whales' ear bones, then animals with legs that swam in shallow water. Together they show how whales evolved from land-dwelling mammals.

Each of these was found in rock layers of the age that evolution predicts.

Why the fossil record has gaps

The fossil record is like a book with most of its pages missing.

• Fossilization is rare: most organisms decay or are eaten.
• Soft-bodied organisms seldom fossilize.
• Many rocks have been eroded, melted or folded by geological processes.
• Many fossils are still buried and have never been found.

Gaps do not weaken the theory. What matters is that every fossil found fits the predicted order, and new finds (like Tiktaalik) keep filling gaps in the expected places.

Careers and Canadian contributions

Palaeontologists, geologists, museum curators and fossil preparators (technicians who free fossils from rock) work across Canada. Most have a university degree in biology or geology, and many have a graduate degree. The Royal Tyrrell Museum in Drumheller, Alberta, studies dinosaurs from Alberta's badlands, one of the richest dinosaur fossil regions in the world. The Joggins Fossil Cliffs in Nova Scotia (a UNESCO World Heritage Site) preserve a forest about 310 million years old with some of the earliest known reptiles, and Mistaken Point in Newfoundland and Labrador (also UNESCO) preserves some of the oldest known complex multicellular life, over 560 million years old.