Beginning of Life
Fish split into two major groups: the bony fish and cartilaginous fish. The cartilaginous fish, as the name implies, have skeletons made of cartilage rather than the harder bone. They eventually include all the sharks, skates and rays.
440 million years ago
The bony fish split into their two major groups: the lobe-finned fish with bones in their fleshy fins, and the ray-finned fish. The lobe-finned fish eventually give rise to amphibians, reptiles, birds and mammals. The ray-finned fish thrive, and give rise to most fish species living today.
The common ancestor of lobe-finned and ray-finned fish probably has simple sacs that function as primitive lungs, allowing it to gulp air when oxygen levels in the water fall too low. In ray-finned fish, these sacs evolve into the swim bladder, which is used for controlling buoyancy.
425 million years ago
The coelacanth, one of the most famous “living fossils” – species that have apparently not changed for millions of years – splits from the rest of the lobe-finned fish.
417 million years ago
Lungfish, another legendary living fossil, follow the coelacanth by splitting from the other lobe-finned fish. Although they are unambiguously fish, complete with gills, lungfish have a pair of relatively sophisticated lungs, which are divided into numerous smaller air sacs to increase their surface area. These allow them to breathe out of water and thus to survive when the ponds they live in dry out.
400 million years ago
The oldest known insect lives around this time. Some plants evolve woody stems.
397 million years ago
The first four-legged animals, or tetrapods, evolve from intermediate species such as Tiktaalik, probably in shallow freshwater habitats.
The tetrapods go on to conquer the land, and give rise to all amphibians, reptiles, birds and mammals.
385 million years ago
The oldest fossilised tree dates from this period.
375 million years ago
Tiktaalik, an intermediate between fish and four-legged land animals, lives around this time. The fleshy fins of its lungfish ancestors are evolving into limbs.
340 million years ago
The first major split occurs in the tetrapods, with the amphibians branching off from the others.
310 million years ago
Within the remaining tetrapods, the sauropsids and synapsids split from one another. The sauropsids include all the modern reptiles, plus the dinosaurs and birds. The first synapsids are also reptiles, but have distinctive jaws. They are sometimes called “mammal-like reptiles”, and eventually evolve into the mammals.