Here is a timeline of our direct ancestors according to science. Note that some of this is in dispute, but more or less it goes like this (the creatures pictured are reconstructed or are like those in our direct line):
4,000 million years ago: Life begins.
3,900 million years ago: The first cell.
2,500 million years ago: The first to use oxygen.
2,100 million years ago: The first eukaryotic cell – the cell design that our bodies still use.
1,200 million years ago: The first to have sex!
900 million years ago: Looks kind of like a sperm cell. Has a tail and can swim.
600 million years ago: The first to have more than one cell, which leads to tissues of specialized cells.
580 million years ago: The first to have nerves, muscles and eyes!
550 million years ago: Looks like a flat worm. The first to have a brain!
540 million years ago: Looks like a worm. Has a heart and something like gills.
530 million years ago: Pikaia: Looks kind of like an eel and swims like one too. The first to have fins and a backbone of sorts. Lives at the bottom of the sea.
505 million years ago: Looks like a lamprey. The first to have a skeleton.
480 million years ago: Looks like a fish. Has a jaw and some scales. Has separate fins at the bottom.
390 million years ago: Still a fish but with strong fins that help it move in shallow water full of leaves and sticks and stuff.
365 million years ago: Looks like a newt. Walks on dry land: it has legs and lungs. Lives both in the water and on land.
300 million years ago: Looks like a lizard. Lives on land.
125 million years ago: Looks like a mouse. Has hair, warm blood and gives birth to live offspring. Its brain has a neocortex. Can climb small shrubs. It only comes out at night: it is the age of the dinosaurs!
55 million years ago: Looks kind of like a squirrel. Lives on the ground and in the lower branches of trees. Eats fruits and leaves. Has claws.
30 million years ago: Looks like a monkey. Lives in trees and awake during the day. Eyes face forwards. Can hold things. Brain: 20 cc.
25 million years ago: An ape. No tail, can see in colour, has a large brain for its size. Brain: 167 cc.
7 million years ago: The last common ancestor of humans and chimpanzees. Brain: 350 cc.
3.5 million years ago: Austalopithecus afarensis: Lucy’s people. Has left the trees to live in the grasslands of Africa. Walks on two feet, eats meat, losing its body hair. The first to make tools. A scavenger, not a regular hunter. Hunted by saber-tooth cats (Dinofelis). Brain: 440 cc.
500,000 years ago: Homo heidelbergensis: The last common ancestor of humans and Neanderthals. Hunts, builds shelters of stone and wood, uses fire, buries his dead. Can talk to a degree. Brain: 1230 cc.
135,000 years ago: Homo sapiens: Mitochondrial Eve, who lives in East Africa. Fully human. Adapts mainly through technology not biology. Brain: 1350 cc.