THE ORIGINS OF HUMANKIND

Paleontologists, scientists who study ancient life in the distant past, agree that humans and chimpanzees are descended from a species that no longer exists. About seven million years ago in central Africa that lost species gave rise to two separate species. One branch developed into chimpanzees, the other, into hominids. After leaving their homeland in Africa around 1.8 million years ago, hominids continued to develop until anatomically modern people, or Homo sapiens sapiens, appeared 150,000 years ago.

The First Hominids: Australopithecines

Biologists use four different subcategories when classifying animals: family, genus (the Latin word for group or class), species, and subspecies. Modern humans are members of the Primate family, the genus Homo (“person” in Latin), the species sapiens (“wise” or “intelligent” in Latin), and the subspecies sapiens, so the correct term for modern people is Homo sapiens sapiens. Members of the same species can reproduce, while members of two different species cannot. The species closest to modern human beings today is the chimpanzee, whose cells contain nuclei with genetic material called deoxyribonucleic acid (DNA) that overlaps with that of humans by 98.4 percent.

Scientists use the concept of evolution to explain how all life forms, including modern humans, have come into being. In the nineteenth century Charles Darwin proposed that natural selection is the mechanism underlying evolutionary change. He realized that variations exist within a species (we now know that genetic mutations cause them) and that certain variations increase an individual’s chances of survival. All variations, benefi cial or not, are passed along to offspring. Because those individuals with benefi cial traits—perhaps a bigger brain or more upright posture—are more likely to survive, they will have more offspring. And, because traits are inherited, these offspring will also possess the benefi cial traits. Individuals lacking those traits will have few or no offspring. As new mutations occur within a population, its characteristics will change and a new species can develop from an earlier one, typically over many thousands or even millions of years.

The process of natural selection caused the early hominids to diverge from other primates, especially in their manner of walking. Our very earliest ancestors did not belong to the genus Homo but to the genus Australopithecus (“southern ape”), whose habitat was also Africa. The defining characteristic of australopithecines was bipedalism, the ability to walk on two feet, whereas chimpanzees and gorillas knuckle-walked on all four limbs.

Paleontologists have found australopithecine remains in different regions of Africa, including Malawi, Chad, South Africa, and most importantly northern Tanzania in the Olduvai Gorge, which lies in the Great Rift Valley. Located along a crack in the earth’s crust where two giant tectonic plates are slowly moving apart, the Great Rift Valley runs for 2,000 miles (3,600 km) from the Red Sea to Tanzania. Now 25 to 50 miles (40–80 km) wide, it exposes many earlier layers of human occupation. In the 1960s and 1970s, Richard and Mary Leakey found in this valley australopithecine skulls, jawbones, teeth, and even footprints, the first tangible evidence of the australopithecine ability to walk.

Since early hominids could walk, they were able to leave the cover of forests and hunt in the open grasslands. Scientists believe that millions of years ago grasslands began to replace forests. At that time the African continent, then much wetter than it is today, began to dry up. As grassy savannah replaced rainforest, walking upright conferred an important advantage over those species that walked on four limbs. Upright walking also used fewer calories than knuckle-walking.

One of the most complete sets of australopithecine remains comes from a female known as Lucy. She was named for “Lucy in the Sky with Diamonds,” a Beatles song playing on a tape recorder when, in 1974, archaeologists found her remains in Ethiopia. She stood 39 inches (1 m) tall and lived 3.5 million years ago. Her face was shaped like an ape, with a small brain like a chimpanzee, and she made no tools of her own. But the remains of Lucy’s knee showed that she walked upright.

Homo Erectus and the First Migrations Outside of Africa, 2.5-1.8 Million Year Ago

One can easily get the impression that our ancestors evolved in a logical progression, with ancient tiny hominids growing taller and more human-like, but the reality was more complex. Fossil evidence suggests that a number of species came into existence, flourished for a time, and then died out. Some of these had more human characteristics, while others had more in common with chimpanzees and gorillas.

Recent finds suggest that four different groups of human ancestors coexisted 2.5 million years ago, or one million years after Lucy was alive. These different human ancestors all shared two important characteristics that set them apart from the australopithecines: they had bigger brains and made tools of their own by chipping off stone flakes from cores. This innovation marked the beginning of the Paleolithic period, or Old Stone Age (2.5 million years ago–8000 b.c.e.). The earliest tools were sharp enough to cut through animal hides and to scrape meat off bones. Over the long course of the Paleolithic period, proto-humans made tools of increasing complexity.

These early hominids belonged to the same genus, but not the same species, as modern people: they are called Homo habilis (“handy human”). Standing erect, Homo habilis weighed less than 100 pounds and measured under 5 feet tall. They ate whatever fruits and vegetables grew wild and competed with other scavengers, such as hyenas, to get scraps of meat left behind by lions and tigers.

The species that evolved into modern humans—Homo erectus  (“upright human”)—appeared about 1.9 million years ago. Homo erectus had a brain double the size of earlier hominid brains, about the same size as that of modern humans, as well as far greater mobility than any earlier human ancestor. Armed with hand axes, and perhaps even simple boats (evidence of which does not survive), Homo erectus left Africa and migrated to Asia.

Two main routes connect Africa with Eurasia: one leads across the Strait of Gibraltar into Spain, and the other crosses the Sinai Peninsula into western Asia. The earliest hominid remains found outside of Africa date to 1.8 million years ago in Dmanisi, Georgia (in the Caucasus Mountains) and in Ubeidiya, Israel, from 1.4 million years ago. The distribution of the few Homo erectus finds in Eurasia shows that the route into western Asia was more heavily traveled than the route across the Mediterranean at the Strait of Gibraltar.

These early species did not move quickly from western Asia to Europe, perhaps because the cold climate there was not inviting. They entered Europe by crossing the Bosporus strait in modern Turkey and then walking along the northern shore of the Mediterranean. The earliest evidence that our ancestors lived in Europe, from Atapuerca, Spain, dates to 1.1 million years ago.

It seems likely that Homo erectus learned how to control fire at this time. One early find, dating to 1.4 million years ago, at the site of Chesowanja, Kenya, included lumps of burnt clay alongside animal bones and stone tools, but many think a natural fire might have occurred. A more convincing find, from northern Israel, dates to 790,000 years ago. There archaeologists found fragments of flint next to charred remains of wood in different layers of earth, an indication that the site’s occupants passed the knowledge of fi re making to their descendants. To keep a fi re going requires planning far ahead, an ability that earlier hominids lacked.

Following the discovery of fire, our ancestors began to eat more meat, which resulted in a larger brain size. By 500,000 years ago, they had settled many sites throughout Europe and Asia. At this time, Homo erectus began to evolve into archaic Homo sapiens.

The Emergency of Homo Sapiens Sapiens, 2 Million-150.000 Years Ago.

One of the fiercest debates currently raging among paleontologists concerns the development of humankind between about 1.9 million years ago, when Homo erectus was alive, and the appearance of Homo sapiens sapiens, or anatomically modern people, about 150,000 years ago. The “regional continuity” school holds that, after Homo erectus settled the Eastern Hemisphere about two million years ago, different hominids from different regions gradually merged to form modern people. The “single origin” school posits a very different history for humankind. It agrees with the regional continuity school that Homo erectus settled the Eastern Hemisphere starting 1.9 million years ago, but it suggests that a second wave of migration out of Africa—of Homo sapiens sapiens between 100,000 and 50,000 years ago—supplanted all pre-existing human species.

The “single origin” school cites DNA evidence in support of its position. The DNA in the nucleus of every cell in our bodies contains about 100,000 genes, an inherited mix of one’s father’s and mother’s DNA. In addition, all of our cells contain structures outside the nucleus called mitochondria. Each of these mitochondria contains a single DNA strand that is inherited only from the egg cell of the mother. If one examines mitochondrial DNA from different populations around the world, statistical analysis of minute differences points toward an ancestral female population that lived somewhere in Africa about 100,000 years ago, an indication that the descendants of an “African Eve” replaced all other human populations after a second migration out of Africa.

The “regional continuity” scholars argue that different regional populations could have mixed with the second wave of migrants from Africa and that they did not necessarily die out. Everyone agrees, though, that the earliest hominids originated in Africa and that Homo erectus left Africa about 1.9 million years ago.

Written by Valerie Hansen and Kenneth R.Custis in "Voyages in World History", Wadsworth, USA,2010, excerpts pp. 5-8. Digitized, adapted and illustrated to be posted by Leopoldo Costa.