Feature: Meet the ancestors
3 October 2006. By Giles Newton and Penny Bailey
In October 2004, the headlines worldwide were full of the sensational discovery of 'hobbit' human skeletons on the Indonesian island of Flores. Dubbed Homo floresiensis, they were just 1 metre tall, had a grapefruit-sized brain, and lived until 12 000 years ago - so would have coexisted with modern Homo sapiens.
Since then, however, controversy has raged over the tiny human. Is it a descendant of an older human species such as Homo erectus, which is known to have lived on nearby Java, or is it a modern human suffering from microcephaly, a disease that causes, literally, 'small brain', or a pygmy? Such quarrels seem almost obligatory between researchers studying human origins: every new discovery or theory sparks argument and counterargument.
The hurdles for researchers are, however, considerable: the fossils they rely on are exceedingly rare and usually incomplete, and it is often difficult to work out which fossils represent which species of early human, how many species there are, their relationships, and when they appear. No wonder palaeoanthropologists argue.
In recent years, the fossil evidence has been joined by insights from genetics and new bioarchaeology techniques. And what they do agree on, mostly, is that human evolution began in Africa, with the split of the human and chimpanzee lineages roughly 7 million years ago (see box, below). Right at the foot of the human family tree is the famous Toumai fossil, unearthed in Chad in 2002. Dating from 6.5-7.5m years ago, the species - Sahelanthropus tchadensis - is regarded as the first with clearly hominid features (although it has also been described as a female ancestor of gorillas).
After that, about 5-6m years ago, come Orrorin and Ardipithecus, which perhaps still resembled upright walking chimpanzees, and then the Australopithecines, widespread throughout eastern and southern Africa from about 4.4m to 1.7m years ago. There were several species of these slightly built and slender (or 'gracile') hominids, the best-known being Australopithecus afarensis, exemplified by 'Lucy', discovered in Ethiopia in 1974. Lucy's relatives are proposed to have left the first definitive evidence of bipedalism - the Laetoli footprints, uncovered by Mary Leakey in 1976-77. Dated to about 3.5m years ago, they show that at least two hominids of this era walked on two legs across wet volcanic ash. The ash hardened like cement and was then covered by more ash.
Unsurprisingly, there are many disagreements about which species should be included in our own genus, Homo. The standard view is that Homo habilis - nicknamed 'handy man' by its discoverer, Louis Leakey - is the first human species, appearing about 2.4m years ago. But some palaeontologists argue that habilis is an Australopithecus, and that the first real Homo is Homo ergaster or erectus (which some regard as one big species but others separate into two).
Appearing about 1.9m years ago, Homo erectus certainly appears the most successful of the early hominids. With bodies much like ours in size and shape (although their skulls are very different and their brains are smaller), they walked upright, used tools such as handaxes and expanded not just out of Africa but across the Old World. They had reached Georgia and even Java by 1.6-1.8m years ago, and may have survived in Java until 30 000 years ago.
Homo erectus is seen as the progenitor of several different human species, includingHomo heidelbergensis, which lived from about 600000 to 250000 years ago in Europe and led to Neanderthals (our closest human relatives; see box, below) and, in Africa, Homo sapiens.
Modern humans
The origin of modern humans has probably been the most contentious issue in the field over the last 20 years. Two models have been put forward. The 'out of Africa' model is the most widely accepted at present, proposing that the action took place in Africa, where Homo erectus or heidelbergensis evolved into Homo sapiens ('sapiens' meaning wise or intelligent). Conversely, the multiregional model proposes that the transition from erectus to sapiens took place in a number of places over a long period of time, and that the intermingling of the various populations eventually led to the single Homo sapiens species we see today.
Genetic studies tend to support the out of Africa model. The highest levels of genetic variation are found in Africa, and modern mitochondrial DNA has been tracked back to just one African woman who lived 10 000 generations ago - 'mitochondrial Eve'.
The Omo I and Omo II skulls, found in 1967, are the oldest known remains of modern humans. That they are dated to 195 000 years ago shows how recent, in evolutionary terms, humans are. These anatomically modern humans did expand out of Africa, and are found in Israel, for example, about 100 000 years ago. But they do not seem to have been particularly successful or numerous; it appears that at times, they teetered on the brink of extinction, dwindling to as few as 10 000 people. The eruption of supervolcano Mount Toba in Sumatra 70 000 years ago may have led to a 'nuclear winter', then a 1000-year ice age. Yet it may be just such environmental catastrophes that placed pressure on humans to cooperate with each other, giving rise to modern human behaviour.
Between 80 000 and 50 000 years ago, humans that were anatomically and behaviourally modern expanded out of Africa. This time, because of their new behaviour, they covered the whole world in a relatively short period of time, encountering and eventually replacing the earlier forms of human who were already there. Genetically, the 6 billion people of the modern world are little different to these adventurous Homo sapiens. Essentially, they are us.
Further reading
- Serre D et al. No evidence of Neandertal mtDNA contribution to early modern humans. PLoS Biol 2004;2(3):e57.
- Bailey P. A series of fortunate events (Wellcome Science, July 2005).
- Pollard KS et al. An RNA gene expressed during cortical development evolved rapidly in humans. Nature 2006;443(7108):167-72.
- Popesco MC et al. Human lineage-specific amplification, selection, and neuronal expression of DUF1220 domains. Science 2006;313(5791):1304-7.
- Khaitovich P et al. Parallel patterns of evolution in the genomes and transcriptomes of humans and chimpanzees. Science 2005;309(5742):1850-4.
External links
- 100 000-year-old DNA sequence allows new look at Neandertal's genetic diversity (News: 7 June 2005, Wellcome Trust Human Genome website)