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Like all other organisms, humans have evolved over time from earlier species, and share a genetic relationship to all other forms of life on Earth. The study of human evolution involves understanding the similarities and differences between humans and other species in their genes, body form, physiology, and behavior. To understand human evolution one must understand where humans fit in relation to other forms of life. Modern humans belong to the group of mammals known as Primates. This is the scientific category describing such diverse creatures as lemurs, lorises, tarsiers, the monkeys of the New World and Old World, and also the apes. As primates we all share many characteristics, such as overlapping fields of vision caused by forward looking eyes (this allows for greater 3D vision), fine ability to grasp and handle objects in our hands, and enlarged brains relative to body size. The evolution of the Primates started in the early part of the Eocene epoch (about 55 million years ago).
By comparing humans and other living species, scientists have learned that humans are most similar to the large apes of Africa and Asia. Among all animals, humans and apes are the most alike in brain and body form, by having a complex social life, and in many other major and minor features, including the lack of a tail. The fossil record of several ancient ape species collectively called Proconsul shows that the split between the common ancestors of the Old world monkeys (above left) and the apes (above right) happened in the earliest Miocene, at least 20 million years ago. Comparisons of DNA show that our closest living relatives are the ape species of Africa, and most studies by geneticists show that chimpanzees and humans are more closely related to each other than either is to gorillas. However, it must be stressed that humans did not evolve from living chimpanzees. Rather, our species and chimpanzees are both the descendants of a common ancestor that was distinct from other African apes. This common ancestor is thought to have existed in the Pliocene between 5 and 8 million years ago, based on the estimated rates of genetic change. Both of our species have since undergone 5 to 8 million years of evolution after this split of the two lineages. Using the fossil record, scientists attempt to reconstruct the evolution from this common ancestor through the series of early human species to today's modern human species. So when did humans originate? The answer to that question really depends on what traits are meant by the term "human." Our understanding of the fossil record shows that distinctively human traits
appeared neither recently nor all at once. Rather, they evolved piecemeal over
a period of roughly 5 million years. By 4 million years ago, humans were habitually
bipedal (walking on two legs) yet had brains roughly a third of the size
of a modern human's (about the size of a modern ape's brain). By 2.5 million
years ago the manufacture of stone tools was common. Large increases in brain
size occurred even later. Complex behaviors such as adaptation to a wide range
of environments and cultural diversification emerged only within the last 100,000
years. Do you need a crash course in anatomy?
Examine the cranium of the modern African ape Gorilla
gorilla, the mountain gorilla, to learn more about the anatomical features
that we will discuss here.
Or, continue on, and follow the human
family tree after it splits from the other African apes.
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