Fossils unlock the evolution of life on Earth, revealing our path from mere microscopic filaments to upright humans. Here’s a quick journey through the ages as told in 10 fossils, adapted from “A History of Life in 100 Fossils” by Paul D. Taylor and Aaron O’Dea (Smithsonian Books, $34.95).
1. Apex Chert
These – the world’s oldest fossils – are estimated to be about 3.465 billion years old. But are they truly fossils? Found in Western Australia in a glassy rock called the Apex Chert, they are made up of microscopic filaments that some scientists believe are nothing more than non-biological, inorganic structures. But others argue the fossils are bits of bacteria, which is consistent with chemical evidence, suggesting that life on Earth did in fact exist 3.5 billion years ago.
2. Doushantuo fossils
Between 560 million and 580 million years old, the Doushantuo Formation in China is remarkable because it preserves a rare example of animals without a hard skeleton. These beautifully preserved fossils of soft-bodied organisms astonishingly resemble early embryos of some modern animals.
Small plants like Cooksonia made the evolutionary leap from the seas to the land four hundred million years ago and dominated the landscape for 40 million years. They set the stage for more complex plants — and eventually the arrival of terrestrial animals — by stabilizing the soil and oxygenating the atmosphere.
Another momentous leap from sea to land was the evolution of fish into tetrapods, or four-legged land-dwelling creatures like amphibians, reptiles, birds, and mammals. Eusthenopteron is an important link in this evolution. It is a fish whose fins are joined to the body by a single bone, a bone that is the equivalent of the limbs found in tetrapods, and it was the tetrapods that spread out across the land, ultimately laying the foundation for the evolution of humans.
Though it looks like a dinosaur, Dimetrodon lived 50 million years before the first dinosaurs walked the Earth. It was closely related to the therapsids, the branch of land vertebrates that included the precursors of all mammals.
Plesiosaura were reptiles with long necks and flipper-like paddles, and they disappeared about 70 million years ago. Their necks were so long and heavy they could never have come out of the water – they needed the buoyancy of water to manage their necks. And here’s a shocker: the Loch Ness monster was not a plesiosaur, despite what an article in the journal Nature suggested in 1975 because, with its insupportable neck, it could not have traveled over land to reach the Loch.
Archaeopteryx is widely hailed as the perfect evolutionary link between birds and reptiles. Some of its features are birdlike (wings, feathers, and a wishbone) while others are reptilian (teeth, a bony tail, and claws on its hands). Therefore this creature shows us a possible connection or continuity between two groups of animals that appear to be so different.
A fisherman made a shocking discovery in 1938: he pulled out of the sea a living coelacanth — a member of a group of fishes that was until then known only from the fossil record, spanning from 70 million to 400 million years ago and presumed to have gone extinct along with the dinosaurs. This “biological find of the century” revealed not only that coelacanths remained relatively unchanged for 400 million years but starkly illustrated how little we know about life in the world’s oceans today.
9. Laetoli footprints
This series of tracks, perfectly preserved by the ashes from an ancient Tanzanian volcano, include some unmistakably hominin tracks. They depict three individuals traveling in the same direction. We know these to be made by an early human because the feet had a big toe in line with the foot (as opposed to ape feet with splayed toes useful for climbing trees) and the heels dug deeper than the toes, implying a fully upright style of walking. These trace fossils are a record of active life with a connection to us.
10. Homo heidelbergensis
Homo heidelbergensis, like this example found in Africa in 1921, is estimated to be between 200,000 and 300,000 years old. Although undeniably human in general appearance, the skull has a huge face and eyebrow ridges, as well as a low forehead. Scientists estimate that Homo heidelbergensis would have had a stature close to that of modern humans, with a skull capacity allowing for a brain only about 14 percent lighter than that of the average modern human. This find has particular importance in human evolution, as many scientists interpret it as the last ancestor of modern humans.