THE LIVING WORLD

Unit Five. Evolution of Animal Life

21. How Humans Evolved

The skull you see above is that of an adult male hominid found in the Amud cave in Israel in early summer of 1961. The cave is perched about 30 meters above a streambed through which water flows to the Sea of Galilee, and by a perennial spring. Modern estimates using electron spin resonance techniques date the hominid occupation of the cave at between 40,000 and 50,000 years ago. Perhaps the water attracted the hominids to the site. Judging from the degree of closure in the sutures (connections) between the cranial bones, the adult male died around age 25, apparently of a blow to the side of the head. He is clearly a Neanderthal, with a long narrow face, pronounced brow ridge above the eyes, and a perfectly round cranium, like a bowling ball. The skull’s age is relatively late in the period when Neanderthals lived, but most remarkable is the size of his brain. Modern humans have brain sizes of about 1,500 cubic centimeters (cc). This individual’s brain is 1,740 cc! While not as large as this, Neanderthal fossils typically have larger brains than modern humans, on the order of 1,650 cc. This raises a very interesting question: As hominids evolved, did their brains become progressively larger? And does this suggest that Neanderthals were smarter than us?

21.1. The Evolutionary Path to Humans

Humans are new arrivals on the biological scene. Fifty years ago, a visual image was proposed that makes this point in a powerful way. Imagine a motion picture of earth taken from space, beginning 757 million years ago, with one image being photographed each year. If you project this film at the normal speed of 24 images per second, it would take you a year to view it, with each day representing 2.1 million years. Starting on January first, there is no visible life on earth’s surface for the first four months. In May the first plants cover the land. Dinosaurs dominate the world for 70 days, from late September to December first, when they disappear abruptly. By late December, the modern families of mammals appear, but not until midday on New Year’s Eve are direct human ancestors seen. Between 9:30 and 10 p.m., Homo sapiens migrates out of Africa to Europe, Asia, and America. At 11:54 p.m., recorded human history begins.

The story of human evolution begins then, around 65 million years ago, with the explosive radiation of a group of small, arboreal (tree-dwelling) mammals called the Archonta. These primarily insectivorous mammals had large eyes and were most likely nocturnal (active at night). Their radiation gave rise to different types of mammals, including bats, tree shrews, and primates, the order of mammals that contains humans.

The Earliest Primates

Primates are mammals with two distinct features that allowed them to succeed in the arboreal, insect-eating environment:

1. Grasping fingers and toes. Primates have grasping hands and feet that let them grip limbs, hang from branches, seize food, and, in some primates, use tools. The first digit in many primates is opposable.

2. Binocular vision. The eyes of primates are shifted forward to the front of the face. This produces overlapping binocular vision that lets the brain judge distance precisely—important to an animal moving through the trees.

Other mammals have binocular vision, but only primates have both binocular vision and grasping hands, making them particularly well-adapted to their environment.

The Evolution of Prosimians and Anthropoids

About 40 million years ago, the earliest primates split into two groups: the prosimians and the anthropoids (figure 21.1). The prosimians looked something like a cross between a squirrel and a cat. Only a few prosimians survive today: the tarsiers, lemurs, and lorises. Most prosimians are nocturnal.

Figure 21.1. A primate evolutionary tree.

The most ancient of the primates are the prosimians, while the hominids were the most recent to evolve.

The anthropoids, or higher primates, include monkeys, apes, and humans. The early anthropoids, now extinct, are thought to have evolved in Africa. Their direct descendants are a very successful group of primates, the monkeys. About 30 million years ago, some anthropoids migrated to South America. Their descendants, the “New World” monkeys, are easy to identify: All are arboreal, they have flat spreading noses, and many of them grasp objects with long prehensile tails. By contrast, the “Old World” monkeys include grounddwelling as well as arboreal species, have dog-like faces, and do not have prehensile tails.

Key Learning Outcome 21.1. The earliest primates arose from small, tree-dwelling insect eaters and gave rise to prosimians and then anthropoids.