THE RISE OF HOMINIDS, THE ROLE OF ROCK, AND FOOD FOR THOUGHT”
10 million to 1 million years ago
|Early bipedal ancestors. Australopithecus africanus from Sterkfontein Cave, South Africa ca. 2.5 million years ago, an example of a small-brained bipedal hominin.
Credit: Photo by Kathy Schick, courtesy of the Stone Age Institute. All rights reserved.
How did our ape-like ancestors come to be human? This time scale documents the transition from our ape-like ancestors to the earliest bipedal hominids. The earlier parts of this story occurred on the African continent during a time of climatic fluctuation and a general cooling and drying phase. By five million years ago human ancestors were walking upright but still had small, ape-like brains. By two-and-a-half million years ago, two major hominid branches had emerged: the smaller-brained, larger-toothed robust australopithecines (destined for extinction), and the larger-brained ancestors of what would become ourselves, modern humans.
The first recognizable stone tools and cut-marked animal bones emerge about two-and-a-half million years ago as well. The earliest documented human technology consists of battered pebbles that were used to flake stone as well as to crack open animal bones, and flaked cobble-cores and the resultant flakes and fragments struck off of them. Cut-marks and percussion fracture marks on animal bones indicate that tool-using hominids were butchering a range of smaller and larger animals and feeding on the meat and marrow. Technology could have also been used to shape and sharpen sticks for digging up edible roots and tubers and for making containers for carrying plant foods and other materials. These earliest technologies (called the “Oldowan” after the famous site of Olduvai Gorge in Tanzania) are contempory with smaller-brained australopithecines, Australopithecus garhi in East Africa and Australopithecus africanus in South Africa. These hominids had brains about one-third the size of modern humans.
By around two million years ago, human ancestors were developing larger brains and smaller jaws and teeth, probably with the habitual use of stone tools and other material culture and technology. Quite likely a feedback system was co-evolving between biology, on one hand, and technology and culture on the other. The earliest forms of our own genus, Homo rudolfensis and Homo habilis, emerged at this time, hominids with larger brains, about one-half the size of that of modern humans. By 1.8 million years ago, humans had spread from Africa to Eurasia.
A new hominid form, Homo erectus, emerged about 1.7 million years ago. This human ancestor had a brain about two-thirds the size of that of a modern human, and similar body proportions and size to modern humans as well. With the emergence of Homo erectus all other major hominid forms, including the robust australopithecines, appear to go extinct. Coinciding with the emergence of this new hominid form is a new technology, called the “Acheulean”, characterized by large cutting tools, handaxes and cleavers, made on large flakes struck from boulder-cores.
THE LAST SHARED ANCESTOR OF APES AND HUMANS
7 million years ago
Genetic evidence suggests that humans and our closest relatives, the African apes (gorillas, chimpanzees, and bonobos or “pygmy chimpanzees”) shared a common ancestor but then went down different evolutionary pathways (an evolutionary “split” or branch) around seven million years ago. The further back we go in the human fossil record, the more these forms are ape-like. Unfortunately, we have very few fossils between 10 and 7 million years ago, so that this hypothetical last common ancestor is still elusive. As previously mentioned, animals that lived in dense, wet tropical forests are not great candidates for fossilization, as these damp and often acid conditions are not conducive to bone fossilization. But the actual event, this divergence between the human and ape line, is a critical one in our evolutionary history.
THE EARLIEST BIPEDAL HOMININS
6 to 4 million years ago
One of the major features that separates humans from the apes is our mode of locomotion (getting around). On the ground, the modern African apes, all members of the taxonomic tribe Paninae walk on all fours (“knuckle walking”), while humans are of the tribe Homininae (“hominin”), and have evolved to walk upright on two legs. This way of getting around is called bipedality. This fundamental shift and icon of humanity can be seen in the fossil record starting around six million years ago. Two fossil localities in Africa have yielded specimens that appear to show hominin features.
“I AM AN APE-MAN”
THE EARLIER AUSTRALOPITHECINES
4.0-2.5 million years ago
The “classic” ape-men, members of the genus Australopithecus, were first discovered and described in the 1920’s in South Africa. Since then, many more sites in South Africa and in the East African Rift have yielded remains of these small-brained (c. 450 cc), upright-walking creatures, the most famous being “Lucy” discovered at Hadar in Ethiopia. These so-called “ape-men” show some features that are ape-like such as a small brain, a jutting lower face, large canines, and curved phalanges (finger and toe bones), along with more human-like traits such as a broad pelvis, an angled knee joint, and jaws and teeth adapted for chewing side-to-side.
“THE BIG CHILL”
THE BEGINNING OF CLIMATIC COOLING
3 to 2 million years ago
Between three and two million years ago, the African continent and the earth in general experienced a gradual climatic shift to cooler, drier conditions. In many places, tropical forests became woodlands, and woodland became grasslands over time. This cooling phase is correlated with the earliest known stone tools and the emergence of the larger-brained hominin, the genus Homo, as well as the so-called “robust australopithecines.” It is also a time of a major “faunal turnover,” a time when many animal lineages went extinct, while others evolve into new species over time. Prehistoric evidence suggests that hominins became more adapted to more open woodlands and grasslands during this period, an adaptation which became characteristic of the human lineage in Africa for much of its subsequent prehistory.
“OUR POOR COUSINS”
THE LATER AUSTRALOPITHECINES AND THEIR EXTINCTION
2.6-1.0 million years ago
There was more than one evolutionary experiment in upright walking. One branch, represented by the genus Homo, led to us. Another branch, represented by the so-called “robust australopithecines”, co-existed with our ancestors for over one-and-a-half million years. These bipedal hominins were characterized by small front teeth and huge cheek teeth, a slightly larger brain than earlier australopithecines (~550 cc), flared cheek bones, and, in males, a bony (sagittal) crest running down the center of the skull for the attachment of massive chewing muscles.
“THE ROLE OF ROCK”
THE EARLIEST STONE TOOLS
2.6 million years ago
“…and we’ll be saying a big hello to all intelligent life forms everywhere…and to everyone else out there, the secret is to bang the rocks together, guys…”
Douglas Adams, 1981, The Hitchhiker’s Guide to the Galaxy, p. 96.
We are a very, very strange species… We are the only animal on the planet that is totally dependent upon tools and technology for our existence. None of us could survive anywhere in the world without some reliance upon various sorts of tools. What is more, our technological behavior is learned rather than instinctive, and is culturally transmitted from generation to generation. Tools have not only shaped our adaptation, but have also shaped our brains and our bodies, over millions of years.
“I’M YOUR HANDY MAN”
THE EMERGENCE OF THE GENUS HOMO AND BRAIN EXPANSION
1.9-1.5 million years ago
It is likely that the earliest known hominins to flake stone tools around two-and-a-half million years ago were members of the small-brained genus Australopithecus, perhaps with a brain size of 450 cc. Within a half a million years of the earliest stone tools, something fascinating happened: some hominids evolved with substantially larger brain sizes, over 600 cc. Scientists named these larger-brained forms Homo habilis (Latin for “Handy Man”, referring to their ability to make and use stone tools) and put them into our own genus. Another form, Homo rudolfensis, had perhaps an even larger brain (~775 cc) but a massive face with large teeth. These forms were probably responsible for much of Oldowan stone technology. There are a number of hypotheses trying to explain why and how brains started enlarging over the course of human evolution.
“BIGGER AND BADDER”
THE EMERGENCE OF HOMO ERECTUS
1.8 million years ago
In 1879 naturalist Ernst Haeckel proposed a hypothetical “missing link” between apes and humans. He called this Pithecanthropus erectus. In 1891 Dutch army doctor Eugene Dubois found fossil human remains in Java that he named after Haeckel’s hypothetical species. Today we know that such proto-human fossils are members of our own genus but are of a different species, which we now call Homo erectus.
“OUT OF AFRICA”
THE EARLIEST EURASIANS
1.8 million years ago
A little over a decade ago, many anthropologists thought that hominin populations did not move out of Africa and into Eurasia until about one million years ago. This view changed radically with the discovery of the prehistoric site Dmanisi in the foothills of the Caucasus Mountains in the Republic of Georgia (formerly part of the Soviet Union). Here, in geological deposits dating to approximately 1.7 million years ago were found simple Oldowan-like stone tools and five fairly complete skulls of hominins with brain sizes of 600-700 cc.
“AXES OF POWER”
THE INVENTION OF THE HANDAXE
1.76 million years ago
Between 1.7 and 1.5 million years ago, stone age hominins began to make new types of tools. By either striking large flake blanks from boulder cores, or by using large river cobbles the began to make crude pointed or oval-shaped objects called handaxes by archaeologists, and sharp, guillotine-bit cleavers. These types of tools are characteristic of the Acheulean industry, which begins to replace the simpler Oldowan industry starting at this time.