Who are we? Where are we from? We can attempt to answer these questions by studying human evolution. Evolution provides a context for understanding the structure and function of our bodies, and even how we behave and think.
ANCIENT ORIGINS
In placing our species within the animal kingdom, it is clear that we are primates—mammals with large brains compared to other mammals, good eyesight, and, usually, opposable thumbs. Primates diverged, or branched off, from other mammal groups on the evolutionary tree at least 65 million years ago, and possibly as far back as 85 million years ago (see below). Within the primates, we share with a clutch of other species—the apes—a range of anatomical features: a large body with a chest that is flattened front-to-back; shoulder blades on the back of the chest, supported by long collarbones; arms and hands designed for swinging from branches; and the lack of a tail.
The earliest apes emerged in East Africa at least 20 million years ago, and for the following 15 million years a profusion of ape species existed across Africa, Asia, and Europe. The picture today is very different: humans represent one populous, globally distributed species, contrasting with very small populations of other apes, which are threatened with habitat loss and extinction.
Unusual Primate
From bush babies to bonobos, lorises and lemurs, to gibbons and gorillas, primates are a diverse bunch of animals, bound together by a common ancestral heritage (see below) and a penchant for living in trees. Humans are unusual primates, having developed a new way of getting around—on two legs, on the ground.
However, we still share many characteristics with the other members of the wider primate family tree: five digits on our hands and feet; opposable thumbs, which can be brought into contact with the tips of the fingers (other primates have opposable big toes as well); large, forward-facing eyes, which allow good depth perception; nails rather than claws on our fingers and toes; year-round breeding and long gestation periods, with only one or two offspring produced per pregnancy; and flexible behaviour with a strong emphasis on learning.
Primate family tree
This diagram explains the evolutionary relationships between living primates. It shows how humans are most closely related to chimpanzees, and that apes are more closely related to Old World monkeys (including baboons) than New World monkeys (including squirrel monkeys). All monkeys and apes are shown to be more closely related to each other than to prosimians (including lemurs and bush babies).
Great Ape
Although we might like to think of ourselves as separate from other apes, our anatomy and genetic makeup places us firmly in that group. Classically, the apes have been divided into two families: lesser apes (gibbons and siamangs) and great apes (orangutans, gorillas, and chimpanzees), with humans and their ancestors placed in a separate family: hominids. But, since genetic studies have shown such a close relationship between the African apes and humans, it makes more sense to group humans, chimpanzees, and gorillas together as hominids. Humans and their ancestors are then known as hominins.
Not only that, but humans are genetically closer to chimpanzees than either humans or chimpanzees are to gorillas. It’s not surprising that humans have been called the “third chimpanzee.”
Human skull
The skull in humans is dominated by a massive braincase, with a volume of 1,100–1,700 cubic centimeters (cc). Its teeth, jaws, and areas of attachment for chewing muscles are small in comparison with other apes. The brow ridges over the eye sockets are subtle and the face is relatively flat.
Chimpanzee skull
Chimpanzees have a relatively small, rounded braincase, accommodating a brain of 300–500 cubic centimetres in volume. The face is relatively large, with a fairly prominent brow ridge and jaws that project forward.
Gorilla skull 
The occipital torus is high on the skull, with a large area for the attachment of strong neck muscles below it. The male gorilla has a massive brow ridge and a large sagittal crest for the attachment of strong jaw muscles. The size of the braincase is 350–700 cubic centimeters.
Orangutan skull 
Like the chimpanzee, the orangutan has a relatively small braincase, with a volume of 300–500 cubic centimeters, and a large face. The skull is extremely prognathic, with strongly projecting jaws. The brow ridge is much smaller than in gorillas or chimpanzees.
Human Closest Relative:
Science has shown that humans and chimpanzees shared a common ancestor some 5–8 million years ago. Comparing ourselves with our closest relative gives us an opportunity to identify the unique features that make us human. Humans have developed two major defining characteristics—upright walking on two legs, and large brains—but there are many other differences between us and chimpanzees. The human population is huge and globally distributed, but we are, in fact, less genetically diverse than chimpanzees, probably because our species is much younger.
Reproduction is quite similar, although human females reach puberty later, and also live for a long time after menopause. Humans live up to 80 years, while chimpanzees may live up to 40 or 50 years in the wild. Chimpanzees live in large, hierarchical social groups, with relationships strengthened by social grooming; humans have even more complex social organization. Furthermore, although chimpanzees can be taught to use sign language, humans are uniquely adept at communicating thoughts and ideas through complex language systems.
Comparing with Chimpanzees:
Some parts of the human skeleton are remarkably similar to that of the chimpanzee: the shoulder and upper arm are almost the same size and shape. Chimpanzees walk on four legs, which makes their lower skeletons quite different from a human’s, with a long pelvis and short, bent legs.
