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What is your body for?

Your legs are millions of years older than your brain

Upright walkers with brains the size of a chimpanzee’s left footprints in Tanzania 3.66 million years ago. The big expansion in brain size came much later, from around 2 million years ago. Drag the slider under the skeleton, from a chimpanzee through three fossils to you, and watch which parts change first.

Marius Comper · 26 September 2026

Five ape and human skeletons in a row, from a book of 1863.
Skeletons of a gibbon, an orangutan, a chimpanzee, a gorilla and a man, from Thomas Henry Huxley’s Evidence as to Man’s Place in Nature (1863), drawn by Benjamin Waterhouse Hawkins. The gibbon is shown at twice its natural size relative to the others.Wellcome Collection, CC BY 4.0, cropped.

Chimpanzeeliving, for comparisonWalking upright on bent hips and knees, with a tall, narrow hip bone, a big toe that grasps and arms longer than its legs.

  1. Chimpanzee (living, for comparison). Walking upright on bent hips and knees, with a tall, narrow hip bone, a big toe that grasps and arms longer than its legs.
  2. Ardipithecus (4.4 million years ago). A shorter hip bone for walking upright on the ground and small canines, but a big toe that still grasped and a foot with no arch.
  3. Lucy (3.2 million years ago). A short, wide pelvis rebuilt for upright walking, an angled thigh bone and, in her species, arched feet with the big toe in line. Six skulls of her species held at most 550 cm³ of brain; a chimpanzee's holds about 384.
  4. Turkana Boy (1.6 million years ago). An early Homo erectus with long legs, short forearms and a body close to ours. Five African Homo erectus skulls from about 1.9 to 1.5 million years ago held 691 to 909 cm³.
  5. You (today). A brain of about 1,336 cm³, 3.5 times a chimpanzee's, a face tucked under it, and a chin.

The chimpanzee is the closest living body to compare with, not our ancestor: its line has been evolving for as long as ours, since a split at least 7 to 8 million years ago by one estimate. Whether the common ancestor looked like a chimpanzee is still argued. At each fossil stop, parts that the bones do not show changing stay in the chimpanzee's form. The drawing is a side view to scale: arm and leg lengths follow measured ratios at the same trunk length; outlines and gait are drawn, not traced.

People have been making this comparison since 1699

In 1699 the London physician Edward Tyson published the anatomy of a young ape brought from Angola, compared part by part with a monkey, an ape and a man. He called it a “Pygmie”; it was a chimpanzee. He ended with two lists: 48 ways in which it resembled a man more than apes and monkeys do, and 34 ways in which it did not.

His second list reads like the contents of this page: a ridge of bone above the eyes, a small thumb, a big toe “set at a distance from the other, like a thumb”, a hip bone “longer, narrower” than a man’s, thirteen ribs a side instead of twelve.

Tyson also got something wrong. He decided his animal was “designed by Nature to walk erect”. Chimpanzees can walk upright, but it costs them, as the numbers below show.

An engraved skeleton of a young chimpanzee standing upright.
Tyson’s engraving of his chimpanzee’s skeleton, standing upright, 1699.Edward Tyson, Orang-Outang, sive Homo Sylvestris, 1699. Public domain.
A small chimpanzee skeleton in a museum case.
The skeleton itself, on display at the Natural History Museum, London, in 2017.Photo: Thomas Quine, CC BY 2.0.

1 · Walking

A body built to walk far on little fuel

For each kilogram and metre, a chimpanzee walking upright uses about 4 times the oxygen you do. Almost every bone between your skull and your heel has been reshaped for walking upright, and the reshaping makes each step far cheaper.

Side by side

Drawn at the same trunk length

At the same trunk length the proportions speak first. A chimpanzee’s arms are longer than its legs. In one study, five chimpanzees and four people weighed about the same (60 and 69 kg on average), but the people stood with their hips twice as high: 92 cm against 46.

Arms as % of legs101–11427 skeletons
Arms as % of legs64–715 skeletons

Head

The skull sits on top of the spine

The hole where the spinal cord leaves the skull, the foramen magnum, sits further forward in you than in any other primate measured, and points down. Your skull balances on the spine; a chimpanzee’s hangs forward of it and is held by neck muscles. Kangaroos and hopping rodents have a forward hole too.

The position and the angle of the hole can change separately, and ape and fossil values overlap, so the hole alone cannot prove that a fossil walked upright.

Lower back

A longer lower back, curved inward

Chimpanzees usually have four lumbar vertebrae, sometimes three; you usually have five. In apes the tall hip bones close round the lowest of them, so the lower back barely bends and the trunk leans forward. Your lower back curves inward and places the weight of the upper body over the hips. In women the curve is reinforced, which helps in carrying a pregnancy.

Lumbar vertebraeusually 4sometimes 3
Lumbar vertebraeusually 5

Pelvis

A hip bone that wraps round the side

A chimpanzee’s ilium is a tall, flat blade. Yours is short and wide and curves round the side of the body, so two buttock muscles, gluteus medius and minimus, sit at the side of the hip. When you stand on one leg they stop the pelvis sagging towards the lifted leg. A chimpanzee walking upright does it differently: it hitches that side up and throws its whole chest over the standing foot, and its steps are about 3 times wider than yours, relative to leg length.

The difference starts in the embryo: the growth plate of the human ilium is turned 90 degrees compared with a chimpanzee’s.

Legs

Straight legs, angled thighs

Ape hamstrings cannot straighten the hip past about 160 degrees, so a chimpanzee walks with bent hips and knees, which gives its muscles poor leverage and makes each step costly. Your hip opens to as much as 200 degrees in a normal stride.

Your thigh bones also slant inward to the knee, by 8 to 14 degrees, which puts your feet under your body. Babies are born without that angle. Walking builds it by age four to six; in a person who never walked, it did not develop.

Hip extensionabout 160°hamstrings’ limit
Hip extension156–200°while walking

Foot

A foot that pushes instead of grasping

A chimpanzee’s big toe sticks out sideways like a thumb, for gripping branches. Yours lines up with the others and is the last part to leave the ground at each step. Your foot is arched along its length and across it; the crosswise arch alone gives more than 40 percent of the foot’s stiffness, the way a banknote stiffens when you curl it.

The arch is not rigid: while the foot is on the ground, people bend the middle of the foot through a wider range than chimpanzees do.

What your walk costs

Oxygen used per kilogram of body weight per metre, measured on a treadmill: people 0.05 ml; chimpanzees 0.21 ml on two legs and 0.19 ml on all fours. That is 76.8 and 68.5 percent less for you, averaged animal by animal. Set your weight and a distance.

You17.5 l
A chimpanzee, on two legs73.5 l
A chimpanzee, on all fours66.5 l

Five chimpanzees (two of them juveniles, aged 6 and 9) and four people; Sockol, Raichlen and Pontzer, 2007. The litres scale the measured cost to your weight; they are not a measurement of you.

The chimpanzees differed a lot. One 33-year-old female walked more cheaply on two legs than on four (0.16 against 0.29 ml), because she kept her legs straighter and her feet on the ground longer. Variation like hers is what natural selection could have worked on.

Cheap walking is one explanation for why our ancestors stood up, not a settled one. A later study found that chimpanzees spend about the same on two legs as on four, so the first upright steps may have saved nothing; the saving grew as legs lengthened and straightened.

When it happened

Ardipithecus ramidus, 4.4 million years ago in Ethiopia, walked upright on the ground but kept a grasping big toe and climbed. A shin bone of Australopithecus anamensis, 3.9 to 4.2 million years old, is already built for walking. Footprints left in wet volcanic ash at Laetoli, Tanzania, 3.66 million years ago show human-like steps on fairly straight legs, though slightly more bent than ours. Lucy, a little under 3.18 million years old, has a pelvis rebuilt for upright walking and the angled thigh bone. Older candidates, Sahelanthropus (6.7 to 7.2 million years) and Orrorin (about 6 million), are argued over: four papers since 2020 have read the same Sahelanthropus thigh bone both ways.

Upright walking did not begin with a march out of the forest onto open grass. The earliest hominins lived in mosaics dominated by woodland, with strips of forest along rivers, and a study of chimpanzees living in such a landscape today suggests that upright walking may first have been useful in the trees. The body built for long-distance running came much later, with Homo.

Two trails of footprints in grey ash.
Casts of the Laetoli footprints, left in volcanic ash in Tanzania 3.66 million years ago, at the Natural History Museum, Vienna.Photo: Wolfgang Sauber, CC BY-SA 4.0.
Chimpanzees on a dirt road; the one on the right walks on two legs.
Wild chimpanzees crossing a road in Uganda: four knuckle-walk, one walks upright.Photo: Mwanzotoursuganda, CC BY-SA 4.0, cropped.

2 · Running

Built to run in the heat

Chimpanzees sprint, but no primate other than us runs for long. From around two million years ago, with early Homo, bodies gained parts that help running far more than walking: springs, stabilisers and a cooling system.

Heel

A spring in the heel

A chimpanzee’s calf muscle runs almost to the heel: its Achilles tendon is about 7.5 percent of the length of calf muscle and tendon together. Yours is 53 to 65 percent. Stretched at each landing and released at push-off, the tendon and the arch of the foot give back energy that muscle would otherwise have to supply.

When the long tendon appeared is argued. The classic account puts it with early Homo; a 2020 study of heel bones infers about 63 percent in Lucy’s species.

Achilles tendon7.5%of calf length
Achilles tendon53–65%of calf length

Buttocks

Buttock muscles that work when you run

Your buttock muscles weigh about 27 grams per kilogram of body weight; a chimpanzee’s about 17. The largest of them, gluteus maximus, barely works when you walk on the flat. It switches on when you run, and stops the trunk pitching forward at each footfall.

Buttock muscles17.2 g/kgof body weight
Buttock muscles27.0 g/kgof body weight

Neck

A band that keeps the head steady

Running mammals such as dogs and horses have a nuchal ligament, a band from the back of the skull down the neck that stops the head bobbing. Chimpanzees do not have one. You do, and the ridge where it attaches first shows on a skull of early Homo, between about 1.6 and 1.9 million years old.

In 2004 Dennis Bramble and Daniel Lieberman listed 26 features of the human skeleton that help in running; none appears in Australopithecus. They suggested that running let early humans chase animals to exhaustion, and in the same paper noted that it “might have been too energetically expensive”. That question is still open.

Same hair, more sweat

You have about as many hair follicles per square centimetre as a chimpanzee; your body hairs are just fine and short. The real change is in sweat glands: on average ten times as many for each patch of skin. Athletes typically sweat half a litre to two litres an hour. Chimpanzees sweat too; in a small 1976 test, two young chimpanzees in the heat also breathed faster, as every other primate tested did. People rely overwhelmingly on sweating; heat-induced panting has been reported in people too, but whether it does much to cool them is disputed.

Sweat glands in one square centimetre of skin

Chimpanzee: on average a tenth of the human density
Human back: about 75

Kamberov and colleagues, 2018, counted glands in skin samples from 7 people (aged 70 to 92), 4 chimpanzees and 8 macaques. The human count is read from their chart; the forehead reaches about 205 per cm². The chimpanzee square is drawn at the paper’s stated average ratio, ten to one.

An engine that runs hotter

People burn about 400 kcal a day more than chimpanzees and bonobos of the same lean body mass, and carry more fat: in one study 23 percent of body weight in men and 41 percent in women, against 8 to 9 percent in captive chimpanzees and bonobos. Despite all the sweating, people turn over 30 to 50 percent less water a day than other apes of the same size, activity and climate.

3 · Throwing and holding

Built to throw hard and hold things firmly

Your shoulder, waist and hand are tuned for two things a chimpanzee rarely does: throwing hard and straight, and pinching an object firmly between thumb and fingers.

Shoulder

A shoulder socket that faces sideways

A chimpanzee’s shoulder socket points upward, suited to hanging and climbing. Yours faces out to the side. Together with a tall waist that lets the hips turn separately from the chest, this lets you cock the arm far back and store energy in stretched tendons and ligaments, like a catapult. In a study of 20 throwers, that stored energy could supply about half the work of the arm’s inward twist: 54 percent, if 90 percent of it comes back.

The combination first appears together in Homo erectus, about two million years ago.

Upper arm

The fastest movement your body makes

As the arm uncoils, the upper arm twists inward faster than any other movement of the human body: on average 4,290 degrees a second in that study, and more than 9,000 in the fastest throwers measured elsewhere. The price is injury. The ligaments of the shoulder and elbow are not built to take the strain again and again, which is why pitchers tear them.

Hand

A long thumb, or short fingers

Measured bone by bone, your thumb is about three-quarters as long as your ring finger; a chimpanzee’s is about half. One analysis argues that it was the chimpanzee’s fingers that grew longer, for climbing, while ours stayed close to the ancestor’s; others disagree. Among great apes, only humans have a separate muscle to bend the tip of the thumb, which gives the strong pinch that holds a stone while you strike it.

Thumb ÷ ring fingerabout 0.48
Thumb ÷ ring fingerabout 0.78

Twenty metres at the measured speeds

Average throwing speeds, shown in real time.

Chimpanzee, male, 3 years
12.4 mph (20 km/h)
Chimpanzee, male, 9 years
12.5 mph (20 km/h)
Chimpanzee, female, about 30, underhand
19 mph (31 km/h)
Human: the minimum to take part
50 mph (80 km/h)

The only measurements of chimpanzee throwing speed we found: three chimpanzees trained at a sanctuary in California, in the supplement to Roach and colleagues, 2013. The fastest single throw was 22.4 mph (36 km/h), by the adult female, underhand. The “20 mph” often quoted for adult males matches her average; no adult male was measured. The people in the study had to reach 50 mph before they could take part.

Hands, tools and a chimpanzee raised as a child

Stone tools are older than our genus: hammers and flakes from Lomekwi, Kenya, date to 3.3 million years ago. Chimpanzees use tools too. In Côte d’Ivoire they crack nuts with stone hammers, and archaeologists have dug up their hammer stones from 4,300 years ago.

The curve of a chimpanzee’s finger bones seems to be inherited rather than built by climbing. A chimpanzee named Suzy, raised as a child in 1930s New York and rarely allowed to climb, still grew finger bones as curved as a wild chimpanzee’s: about 43 degrees, against about 24 in people.

How much stronger is a chimpanzee?

Muscle fibre for fibrethe same
Muscle for muscle (model)1.35×
Whole body, per kg: average of tests1.5×
Whole body, per kg: range of tests1.2–2.05×

Chimpanzee against human; O’Neill and colleagues, 2017, who also reviewed the tests since 1923. We found no study behind the popular “five to eight times stronger”. It probably grew from tests in the 1920s in which one of two zoo chimpanzees pulled 9.4 times its own body weight on a dial later suspected of being miscalibrated. A 1943 study of eight chimpanzees found them 1.2 times as strong as people per kilogram.

Muscle fibres from chimpanzees are no stronger than ours. The difference is the mix: about two-thirds of the fibres in a chimpanzee’s hip and leg muscles are the fast, powerful type. In the same muscles, 53 to 69 percent of a person’s fibres are the slow type, which tires less. That is a trade of power for endurance.

Teeth

Small canines, very early

Male chimpanzees carry long upper canines, kept sharp against the lower teeth. In Ardipithecus, 4.4 million years ago, male canines were only 6 to 13 percent bigger than females’, within the human range. In chimpanzees and orangutans the difference is 20 to 40 percent. Canines shrank early, at about the time upright walking appears.

Face

A face pulled in under the brain

Chimpanzees spend 37 percent of their waking day eating; people 4.7 percent. A primate of your size would be expected to spend 48 percent. The large chewing face shrank as food got easier to eat. If a third of the diet was meat, eating would have taken nearly two million fewer chews a year; slicing the meat with stone tools cut about 5 percent more, before cooking became common.

Time spent eating37%of the waking day
Time spent eating4.7%expected: 48%

Brain

Three and a half times the brain

About 384 cm³ against 1,336. It is costly: in an adult the brain uses about a fifth of the body’s resting energy, against less than a tenth in a chimpanzee, and in a four-year-old it takes 66 percent.

Brain384 cm³8.6% of resting energy
Brain1,336 cm³19.6% of resting energy
Brain size against timeEach dot is one fossil skull. From 3.2 to about 2 million years ago all are below 560 cubic centimetres, near the chimpanzee range; later skulls rise to 1,390.chimpanzee todayhuman today4008001,2001,60043210million years agocm³upright on the groundLaetoli footprintsAL 162-28, 384 cm³AL 288-1, 388 cm³AL 333-105, 371 cm³AL 333-45, 492 cm³AL 444-2, 550 cm³AL 822-1, 385 cm³Atapuerca 5, 1,125 cm³Atapuerca 4, 1,390 cm³Bodo 1, 1,228 cm³BOU-VP-12/130, 450 cm³BOU-VP-2/66, 995 cm³Bukuran, 916 cm³Ceprano 1, 1,057 cm³D2280, 783 cm³D2282, 648 cm³D2700, 600 cm³D3444, 638 cm³D4500, 546 cm³Hexian 1, 1,025 cm³KNM-ER 1470, 756 cm³KNM-ER 1590, 788 cm³KNM-ER 1805, 582 cm³KNM-ER 1813, 496 cm³KNM-ER 3732, 738 cm³KNM-ER 3733, 781 cm³KNM-ER 3883, 795 cm³KNM-ER 42700, 691 cm³KNM-OL 45500, 715 cm³KNM-WT 15000, 909 cm³Lantian, 780 cm³MH1, 420 cm³MLD 1, 510 cm³MLD 37/38, 445 cm³Mojokerto 1, 800 cm³Nanjing 1, 873 cm³Ndutu, 1,098 cm³Ngandong I, 1,130 cm³Ngandong IX, 1,135 cm³Ngandong V, 1,281 cm³Ngandong VI, 1,097 cm³Ngandong VII, 1,047 cm³Ngandong X, 1,114 cm³Ngandong XI, 1,070 cm³Ngandong XII, 1,145 cm³OH 12, 691 cm³OH 13, 649 cm³OH 16, 638 cm³OH 24, 580 cm³OH 7, 685 cm³OH 9, 1,062 cm³Saldanha 1, 1,225 cm³Sangiran 10, 890 cm³Sangiran 12, 963 cm³Sangiran 17, 982 cm³Sangiran 2, 811 cm³Sangiran 3, 975 cm³Sangiran 31, 700 cm³Sangiran 4, 894 cm³Sangiran 38, 875 cm³Sts 19/58, 509 cm³Sts 5, 481 cm³Sts 60, 413 cm³Sts 71, 442 cm³Stw 505, 558 cm³Taung-1, 428 cm³Tjg-1993.05, 858 cm³Trinil 2, 901 cm³Type 2, 494 cm³UA 31, 995 cm³Yunxian, 1,150 cm³Zhoukoudian II, 1,013 cm³Zhoukoudian III, 915 cm³Zhoukoudian V, 1,300 cm³Zhoukoudian VI, 850 cm³Zhoukoudian X, 1,235 cm³Zhoukoudian XI, 1,018 cm³Zhoukoudian XII, 1,025 cm³
Each dot is one fossil skull, placed at the middle of its date range (Du and colleagues, 2018, a compilation of skulls whose dates fall between 3.2 and 0.5 million years ago; Paranthropus, a side branch with huge jaws, is left out). For more than a million years after the Laetoli footprints were made, every skull held under 560 cm³. The bands show chimpanzees and people today.

How was it paid for? Eating raw food the way apes do, a brain the size of yours would need more than 9 hours of feeding a day, by one model: longer than any ape spends. Easier food is part of the answer. Your gut is also small for your size, with more than half of its volume in the small intestine; in apes the colon is the largest part. The popular idea that a shrinking gut paid for the growing brain did not hold up in a test across 100 mammal species.

Stomach acid like a scavenger’s

Your empty stomach sits at about pH 1.5. In a comparison of birds and mammals, animals that live on carrion averaged 1.3 and omnivores 2.9; a baboon measured 3.7. Each step down the pH scale is ten times more acid, so yours is about 160 times as acidic as the baboon’s. Acid kills microbes, and rotting meat carries plenty, so one explanation is that our ancestors ate more carrion than is usually assumed. Another is that strong acid protects against food-borne infections whatever the diet. And the comparison has a hole: when the study was published, no stomach acidity had been measured for any ape other than us, so nobody knows when the difference arose.

What we lost to talk

A low larynx is often called the key to speech. A chimpanzee’s larynx also descends in infancy, and a model built from X-ray film of a macaque’s moving vocal tract could synthesise an intelligible “Will you marry me?”. The monkey’s throat is not the main obstacle; control from the brain is. What changed in the human throat is a loss. Every other primate examined has thin membranes on the vocal cords; we don’t, and without them the voice is steadier.

The whites of your eyes

Human eyes show more white than any ape’s, and it is the most evenly white. It is not unique: 42 of 51 bonobos in one sample had whites lighter than the iris, while 48 of 50 chimpanzees had them darker. Whether chimpanzee eyes stand out or hide where they look is disputed by two recent studies.

Four primate skulls in a row, from large to small.
Skulls of a human, a chimpanzee, an orangutan and a macaque, to scale. The chimpanzee’s face juts forward beneath a low braincase; the human face sits under a high, rounded one.Photo: Christopher Walsh, Harvard Medical School, CC BY 2.5.

The price of a big head: birth and a long childhood

A human baby is heavy for its mother and its head is a tight fit, yet its brain is less developed at birth than a chimpanzee’s.

Newborn weight, as a share of the mother’s

Chimpanzee3.3%
Human6.1%

Newborn brain, as a share of adult size

Chimpanzeeabout 40%
Humanabout 28%

DeSilva, 2011: 47 captive chimpanzee mother–infant pairs and 2,607 human pairs from the Philippines; brain shares computed from his table of newborn and adult brain sizes.

Human pregnancy lasts 38 to 40 weeks; a chimpanzee’s about 32. Why human babies are born so helpless is argued. The classic answer, the “obstetric dilemma”, is that a pelvis narrow enough for walking cannot pass a bigger head. A 2012 study replied that the mother’s energy sets the limit: by nine months a pregnancy is close to the most a body can sustain, and human pregnancy is already 37 days longer than expected for our size. Later reviews defend a revised dilemma. A tight fit is real: in poorer countries obstructed labour is estimated to cause 8 to 17 percent of all maternal deaths.

Chimpanzees share more of human birth than once thought. In three filmed births the baby came out facing away from its mother and turned as it emerged, as human babies do. In humans the turn happens inside the birth canal and is forced by its shape; one explanation for that shape is that it helps the pelvic floor carry the weight of the organs above it.

Human babies also arrive fat, about 15 percent of their weight, where rats, pigs, sheep and bears are born with 1 to 4 percent. Their brains keep growing fast after birth, and the fat is thought to be a store for it.

Two life histories, by ageChimpanzeeweaned 5first birth 14.330Human foragerweaned 2.5first birth 19.7540102030405060
Averages for wild chimpanzees and for human hunter-gatherers (Kaplan and colleagues, 2000). The end of each bar is the average age at death of those who reach 15.

Human foragers wean at about 2.5 years, chimpanzees at about 5, so human mothers have babies closer together (41 months apart against 67) while still caring for older children, with help from others. At 15, a forager can expect 39 more years, a wild chimpanzee 15. Menopause is not unique to us either: at Ngogo, in Uganda, female chimpanzees live about a fifth of adult life past the age when they can reproduce; hunter-gatherer women about two-fifths. Other chimpanzee communities studied show almost none.

The part that may not be for anything

One difference appears to belong to modern humans alone, among living apes and fossil humans alike: a bony chin. Explanations have been offered for more than a century; a 2016 review concluded that every one of them has “theoretical and/or empirical shortcomings”. One test found that as children grow and the chin becomes more prominent, the jaw does not get stiffer against the forces of chewing; it gets weaker against vertical bending.

In January 2026 a study tested whether the chin itself had been selected for, by comparing the rate of change in jaw and skull shape across the ape family tree. Of nine chin traits, only three showed direct selection. The authors concluded that the chin probably arose largely as a by-product of other changes, smaller front teeth and a reshaped face and skull, and not as an adaptation of its own. That does not settle the argument, but it makes a strange answer the likeliest one: the chin may not be for anything.

Most of your body can be read. The arch of the foot, the band at the back of the neck, the sideways shoulder socket and the small face each point to a job. The chin is a reminder that not every shape has one.

What this comparison cannot tell you

The chimpanzee is not the ancestor. Chimpanzees have been changing for as long as we have. The team that described Ardipithecus argues that the common ancestor was a careful climber, neither a knuckle-walker nor a hanger; others read the same bones, and newer ones, as signs of a chimpanzee-like ancestor. Where the ancestor was different, a trait shared by chimpanzees is not automatically the starting point.

How different are the genomes? Lined up letter by letter where they align, human and chimpanzee DNA differ at about 1.2 percent of positions, about 1 percent once differences within each species are set aside. Complete ape genomes published in 2025 show that 12.5 to 27.3 percent of an ape genome does not line up simply with another’s, so the real gap depends on what is counted. By one estimate from chimpanzee generation times, the lines split at least 7 to 8 million years ago; earlier DNA estimates were 4 to 6 million.

Small samples. Several of the numbers on this page come from a handful of animals: five chimpanzees on a treadmill, three throwing, skin from four. They are the measurements that exist.

A museum case with five ape and human skeletons standing side by side.
Skeletons of an orangutan, two gorillas, a chimpanzee and a human at the Museum of Zoology, University of Cambridge.Photo: DeFacto, CC BY-SA 4.0.

Sources and method

The skeleton is drawn in side view. At the same trunk length, the limbs follow measured arm-to-leg ratios for chimpanzees and people; skull, pelvis and foot outlines are drawn from anatomical references and blend part by part between the two forms. The fossil stops change only the parts the cited fossils show. Walking motion is illustrative. Litres of oxygen in the walking calculator are the measured cost per kilogram and metre multiplied by the weight and distance you choose. Brain shares of adult size and the brain-size chart are computed from the published tables. Numbers read off published charts are marked “about”. Phrases such as “built to” describe what a feature does mechanically; they do not by themselves mean that natural selection produced it for that job.

Pictures: Wikimedia Commons, with author and licence under each image.