The one picture you need to fix first
Most people imagine human evolution as a line. On the left, a hunched ape. On the right, a modern person. In between, a neat row of figures, each one taller and less hairy than the last.
That picture is wrong. It is the most common mistake about this whole subject.
The real shape is a bush. Branches split. A few keep growing. Most stop.
Scientists call these human-like species hominins. For nearly all of the last 7 million years, more than one kind of hominin walked the Earth at the same time. Today only one is left. That is the strange part, not the normal part.
A timeline has to list species in an order, because a page runs from top to bottom. Do not read that order as a queue. Each species below is a twig on a bush, and many of them were alive together.
About 7 million years ago: Sahelanthropus tchadensis
The oldest fossil usually placed near the human branch is Sahelanthropus tchadensis. It comes from Chad, in west-central Africa, and dates to between 7 and 6 million years ago.
Its brain was small, even a little smaller than a chimpanzee’s. But the hole where the spinal cord enters the skull sits further forward than it does in an ape. In a four-legged animal that hole points backwards. In an upright walker it points down. That detail is why some scientists think Sahelanthropus already stood upright.
Other scientists disagree. That argument is still open. Open arguments are how science works, and honest pages should say so.
About 4.4 million years ago: Ardipithecus
In 1994, researchers in the Middle Awash region of Ethiopia found a partial female skeleton. The team studied it for 15 years and described it fully in 2009. They nicknamed her Ardi. Her species is Ardipithecus ramidus, and she lived about 4.4 million years ago.
Ardi is interesting because she is a mix. Her big toe still stuck out sideways for gripping branches, like an ape’s. But her pelvis was shorter and broader than an ape’s, which fits walking on two legs. She lived in woodland, not open grassland.
That last point mattered. Many scientists had assumed upright walking started when forests thinned into grassland. Ardi suggested it may have started among trees instead.
3.85 to 2.95 million years ago: Australopithecus afarensis
This is the species most people have heard of, because of one skeleton.
In 1974, a team working at Hadar in Ethiopia found about 40 percent of a single individual. The catalogue number is AL 288-1. The nickname is Lucy. She lived about 3.2 million years ago.
Lucy’s knees, hips, and spine show she walked upright on the ground. Her arms and shoulders show she still climbed. Her brain was small, usually under 500 cubic centimetres in this species, roughly a third the size of a modern human brain.
There is a second piece of evidence that needs no bones at all. At Laetoli in Tanzania, a team led by Mary Leakey uncovered a trail of footprints pressed into volcanic ash about 3.6 million years ago. The prints show a heel-first, upright walk with a forward-pointing big toe.
So the order is settled: upright walking came first, by millions of years. Big brains came much later.
2.4 to 1.4 million years ago: Homo habilis
Homo habilis lived from about 2.4 to 1.4 million years ago. The name means “handy man”. Its braincase was bigger than Lucy’s, around 600 cubic centimetres, though the face and jaw were still large.
Simple sharp-edged stone tools appear in Ethiopia by about 2.6 million years ago. Someone was striking flakes off pebbles to get a cutting edge. That is a huge step, because a cutting edge lets a slow, weak, clawless animal get at meat and marrow.
About 1.9 million years ago: Homo erectus leaves Africa
Homo erectus appears about 1.89 million years ago and survives until roughly 110,000 years ago. Stop and notice that number. This species lasted well over a million and a half years. Our own species has managed about 300,000 years so far.
Homo erectus is generally considered the first human species to expand beyond Africa. Fossils from Dmanisi in the country of Georgia are about 1.8 million years old, so the spread happened early.
The best single skeleton is nicknamed Turkana Boy, catalogue number KNM-WT 15000, found in 1984 at Nariokotome in Kenya. He was a youngster who died about 1.5 million years ago. His body proportions are strikingly like ours: long legs, short arms, a narrow trunk built for walking and running long distances.
Brain size grew across the long run of this species, from around 700 cubic centimetres in early specimens to over 1,000 in later ones.
700,000 to 200,000 years ago: Homo heidelbergensis
Homo heidelbergensis lived roughly 700,000 to 200,000 years ago, in Africa and in Europe. It had a larger braincase than Homo erectus and a heavy brow ridge.
Many scientists think populations of this general type were close to the ancestors of both Neanderthals and us. The African populations led one way, the European ones another.
One find shows how capable they were. At Schöningen in Germany, archaeologists recovered wooden throwing spears about 300,000 years old. Wood almost never survives that long. These did, because waterlogged mud kept oxygen out. They are carefully shaped, balanced hunting weapons.
400,000 to 40,000 years ago: Neanderthals
Neanderthals lived across Europe and western Asia from about 400,000 to 40,000 years ago. The first recognised specimen came from a limestone quarry in the Neander Valley in Germany in 1856, three years before Darwin published his book.
Their brains were as large as ours and often larger, matched to heavier bodies. They made stone tools, controlled fire, hunted large animals, and buried some of their dead.
The old cartoon of a stupid brute is not supported by the evidence.
Denisovans: a species found mostly in DNA
Denisovans are unusual. We know them mainly from their genome, not their skeletons.
In 2010, researchers sequenced DNA from a small finger bone found in Denisova Cave in the Altai Mountains of Siberia. The DNA did not match Neanderthals and did not match modern humans. It belonged to a third group. Only a handful of physical fossils have been assigned to them since, including a jawbone from Xiahe on the Tibetan Plateau.
This is worth pausing on. A whole human population was identified from a fingertip.
About 300,000 years ago: Homo sapiens
In 2017, a team led by Jean-Jacques Hublin published fossils from Jebel Irhoud, a cave site in Morocco, in the journal Nature. The fossils are about 300,000 years old and are now considered the oldest known members of our species.
That result did two things. It pushed the origin of Homo sapiens back by roughly 100,000 years. And it moved the map. Morocco is in North Africa, not East Africa, so early Homo sapiens were already spread widely across the continent.
The Jebel Irhoud skulls are a mosaic. The face, forehead, and jaw look much like ours. The braincase is long and low, more like older species. Modern faces arrived before modern skull shapes.
A modern human brain averages about 1,350 cubic centimetres. But do not turn brain size into a ranking. Neanderthal brains were bigger than ours, and brain size across living people varies a lot without tracking ability.
Several human species, all at once
About 50,000 years ago, the Earth held at least four kinds of human at the same time:
- Homo sapiens, spreading out of Africa
- Neanderthals in Europe and western Asia
- Denisovans in Asia
- Homo floresiensis, a small-bodied species found on the Indonesian island of Flores in 2003
Add Homo naledi, announced in 2015 from the Rising Star cave system in South Africa and later dated to between about 335,000 and 236,000 years ago, and the bush gets busier still.
This is the strongest argument against the ladder picture. A ladder has one rung at each height. Reality had several branches side by side.
The branches met, and DNA proves it
Here is the part that surprises people most. Those species did not just coexist. They interbred.
People whose recent ancestry lies outside Africa carry roughly 1 to 2 percent Neanderthal DNA. Comparisons of modern genomes with the Neanderthal genome show these stretches sitting in our chromosomes today. Some populations in Melanesia and New Guinea also carry a few percent of Denisovan DNA.
So the Neanderthals are not entirely gone. Fragments of them are inside a large fraction of living people.
Why the ladder keeps coming back
The ladder picture survives because it is easy to draw and easy to remember. It also flatters us, because it puts humans at the top of a climb.
Evolution has no top. It has branches. Every species alive today, including us, sits at the tip of its own twig after exactly the same amount of elapsed time.
Where to go next
If you want to see why humans did not descend from modern monkeys, read did humans evolve from monkeys. To see how these dates are measured in rock, read fossil evidence. To see how the Neanderthal DNA result was obtained, read genetic evidence.