Trends in human biological evolution

Hominids and hominins

• Hominids (the family Hominidae) are the great apes and humans: orangutans, gorillas, chimpanzees, bonobos and humans, plus their extinct relatives.
• Hominins are the narrower group: modern humans and every extinct species on our side of the split from the chimpanzee line, about 6 to 7 million years ago.
• So a chimpanzee is a hominid but not a hominin, and Australopithecus afarensis is both.
• Key hominins in order: Australopithecus afarensis (about 3.9 to 2.9 million years ago), Homo habilis (about 2.4 to 1.4 million), Homo erectus (about 1.9 million to 110 000 years ago), Homo neanderthalensis (about 400 000 to 40 000) and Homo sapiens (from about 300 000 years ago).
• Bipedalism came first; large brains came much later, so the trends did not all happen together.

Bipedalism: skull and spine

• The foramen magnum, the hole where the spinal cord leaves the skull, moved forward to sit centrally under the skull, so the head balances on top of an upright spine instead of hanging forward.
• That needs smaller neck muscles, so the bony ridges at the back of the skull where they attach are smaller in hominins.
• The human spine has an S-shape, with an inward (lumbar) curve in the lower back that brings the trunk's weight over the hips; an ape's spine is a single C-shaped curve.
• Lumbar vertebrae are larger and wedge-shaped in hominins, because they carry the whole upper body's weight when upright.

Bipedalism: pelvis, femur and knee

• The hominin pelvis is short, broad and bowl-shaped, which supports the guts from below and lowers the centre of gravity; the ape pelvis is long and narrow.
• The blades of the ilium curve round to the sides, so the gluteal muscles can hold the pelvis level while the body stands on one leg in each stride.
• The femur angles inwards from hip to knee (the carrying angle), putting the knees and feet under the body's centre of gravity, so the body does not sway side to side as an ape's does.
• The knee joint is larger and can lock straight, so the leg works like a pillar and standing costs little muscle energy; the femoral head is larger too, to carry all the weight on two legs.
• Legs became long relative to arms, lengthening the stride.

Bipedalism: the foot

• The big toe lines up with the other toes and cannot grasp, so it can push the body off the ground at the end of each step.
• Arches along and across the foot act as springs that absorb shock and return energy; an ape's foot is flat and flexible, for grasping branches.
• A large heel bone takes the impact of each heel strike.
• Footprints preserved in volcanic ash at Laetoli in Tanzania, about 3.6 million years old, show an arch and an in-line big toe, so hominins were walking upright by then.

Try it: ape or modern human?

Why bipedalism was selected

• Advantages: walking on two legs uses less energy than knuckle-walking over long distances, frees the hands to carry food, tools and infants, lifts the eyes above tall grass to spot predators, and exposes less of the body to the overhead sun while catching more breeze, which helps keep cool.
• Disadvantages: slower sprinting and poorer climbing, more strain on the back, knees and hips, and a narrower birth canal that makes birth difficult.
• Selection pressures thought to favour it include a cooler, drier climate in East Africa that replaced continuous forest with open woodland and grassland, so food was more spread out and long walks between trees were needed.
• Individuals that walked upright more efficiently found more food and kept cooler, so survived and reproduced more, and the alleles behind those skeletal features increased.

Try it: advantage or disadvantage of walking upright?

Trends in the skull and teeth

• Cranial capacity roughly tripled from australopithecines to modern humans, and the skull became higher and rounder, with a near-vertical forehead.
• The face became flatter (less prognathism), and the jaw and teeth smaller, as food was cut with tools and cooked rather than chewed hard.
• The tooth rows changed from a U-shape with parallel sides to a curved, parabolic arch; canines shrank to the size of the other teeth, so the gap (diastema) that ape jaws need for the opposite canine disappeared.
• Brow ridges became smaller, and Homo sapiens alone has a chin; the bony sagittal crest that anchors large chewing muscles in gorillas and Paranthropus is missing in Homo.

The hand: precision and power

• The human thumb is long, strong and fully opposable: it can rotate to meet the tip of every finger, which gives the precision grip used to hold a needle, a pen or a small stone flake.
• The power grip wraps the fingers and thumb around an object, such as a hammerstone or a spear shaft; both grips need a powerful thumb muscle (flexor pollicis longus) that apes barely have.
• Hominin fingers became shorter and straighter with broad fingertip pads; A. afarensis still had long, curved fingers, a sign it still climbed trees.
• Freed from walking, the hand could be selected for making and using tools, and better tool use selected for a better hand and a bigger brain to plan with it: a positive feedback between biology and culture.

What Achieved, Merit and Excellence look like

• Achieved describes the feature; Merit explains how its structure helps bipedal walking; Excellence links the change to its function, the advantage it gave, and the selection pressure behind it, comparing with an ape.