Evolução humana: uma história evolucionária ainda não contada cientificamente

terça-feira, abril 26, 2011

The untold story of evolution

Around six million years ago in Africa, human history began. But how exactly did hairy, tree-dwelling apes, become modern 21st-century people?

Tim Radford
The Guardian, Monday 25 April 2011
Article history

We share almost 99% of our DNA with chimpanzees. Photograph: Paul Lovelace/Rex Features

Human evolution must be the greatest story never told. It begins in an unknowable past and continues mysteriously for the next five or six million years. Is it a thriller, an epic or a comedy of errors? There is no dust jacket, no title page, no dedication, no acknowledgements. Almost all the text is missing, apart from the occasional phrase, sentence or paragraph, seemingly torn at random from the great six-million-year narrative. If the story of humanity is a single volume, then only the last page survives.

Every so often, scholars find yet another fossilised scrap of the missing narrative, a new character enters, and the plot takes a new twist. Some things are clear: the story began in Africa, between 5m and 7m years ago, with the last common ancestor of two kinds of chimpanzee and ofHomo sapiens sapiens. Charles Darwin calculated as much when he began telling the story in The Descent of Man (1871). "We thus learn that man is descended from a hairy, tailed quadruped, probably arboreal in its habits, and an inhabitant of the Old World," he wrote.

Anthropologists agree on the human-ape connection. The consent is there in the titles of books published in the past 40 years: The Aquatic Ape, The Naked Ape, The Third Chimpanzee, The Talking Ape, Our Inner Ape, The Thinking Ape, The Monkey in the Mirror, The Hunting Apes, The Ape that Spoke and The Artificial Ape.

These books are all attempts to work backwards, from what we are now to what we might have been. The fact that zoologists, anthropologists and palæontologists can write so many books with the word "ape" in the title tells us two things. One is that the evidence is so sparse that people are free to frame a favourite hypothesis about what it was that made humans different. The other is that the human-chimpanzee connection is so clear that there is nowhere else to begin.

First, the family likeness: chimpanzees struggle for status, vocalise, communicate, play politics, use subterfuge, show aggression, reject outsiders, groom and support each other, betray each other and resort to violence or sexual bribery to get their way. Chimpanzees display awareness of self, ability to reason, and a grasp of numbers. Chimpanzees are opportunistic omnivores that also make and use tools for gain, and groups of chimpanzees in the wild have separate traditions, practices and ways of doing things that they pass down the generations. That is, chimpanzees have culture. Chimpanzees and humans have a genetic kinship so close that they share almost 99% of their DNA.

The Victorians called them "man-like apes". Twentieth-century scientists and observers started referring to humans as naked apes. Early in the 21st century, some taxonomists and conservationists began a campaign to change the chimpanzee genus from Pan to Homo, so close are the parallels between the species.

But the African chimpanzee is an endangered species, down to perhaps 150,000, while the human population is about to tip seven billion. The implication is that, long ago, the earliest human ancestors also lived in small social groups, and co-operated and competed for the resources of the woodland and the savannah. Why did humans become so different: bipedal, upright, hairless, with limited strength, feeble jaws, bad backs, embarrassingly large heads and brains with a cerebral cortex four times the size of a chimp's?

For decades, the conventional evolutionary lineage was a simple one: shambling simian stands upright, evolves into bipedal hairy brute, then slouching hairy brute with hand axe and finally into hairless human with BlackBerry. This is the ladder theory of human evolution. It was kicked away long ago. Discoveries in Africa – a femur here, a fragment of skull there, a pelvis, now and again a partial skeleton, a set of footprints fossilised in ancient volcanic mud – reveal a picture more of confusion than direction: a flowering of creatures more or less apelike or manlike, some of them possibly direct ancestors, some of them probably cousins along a parallel lineage, all of them trying to make a subsistence living in a very different Africa, millions of years ago. The fossils turn up in South Africa, East Africa, Ethiopia and even the Sahel. They have generic names such as Sahelanthropus, Ardipithecus, Orrorin, Australopithecus,Paranthropus, and Kenyanthropus, and their remains were unearthed from the dust, stone and mud sediments laid down 3m, 4m and 5m years ago.

Two million years ago, creatures that bear the generic name Homo begin to appear in the fossil record: Homo habilis, Homo ergaster, Homo erectus, and with them appear worked stone tools, hand axes, things for chipping and cutting. Hardly any of these early human relics is complete. Palaeoanthropologists were once fond of saying that the entire human fossil record could be laid out on one table, or packed in a matching set of Gucci luggage, but this is no longer true. What is true is that even 2m years ago, the human lineage begins to look like a bush, with species sprouting in all directions.

And then the story starts to get really complicated. At some point, early humans get up and start moving. They spread. They pack their hand axes, leave Africa and start to colonise the Middle East, Europe, and South Asia. And there is more than one migration out of Africa: firstHomo erectus or something even more primitive, and then, much later,Homo sapiens. And they continue to differentiate into new species. At one point in human history, around 40,000 years ago, modern humans must have shared the planet with at least four other human cousins:Homo erectus, the Neanderthals, a strange, small-brained human found only on the island of Flores in Indonesia, affectionately known as the Hobbit; and most recent of all, species X: a separate human genetic lineage identified in 2010 only by DNA extracted from a finger bone found in a Siberian cave.

What gave early humans their get-up-and-go? Why did humans develop large brains and long legs? Should the first mobile humans be classed as asylum seekers, driven from their native land by climate change? Or were they economic migrants, on the lookout for better opportunities in wide-open Europe and Asia?

Brains are what biologists call expensive items: the human brain at rest consumes 20% of the daily calorific intake. In other words, brains have to be fed. So a large, greedy brain becomes valuable only if it helps to deliver even more food and greater security. So was the larger brain a genetic mutation that increasingly delivered a selective advantage in the struggle for survival? And how did humans get from thinking about food-gathering strategies to thinking about taxonomy, tax-avoidance and Twitter?

The big brain story may have begun in the trees. Arboreal primates that search over wide areas for food in the canopy seem to know what is good for them: they often ignore easy supplies and go looking for special foods. They seem to have a notion of a balanced diet – protein-rich leaves and high-calorie fruits and not too much fibre – and they have been watched deliberately selecting plants with medicinal properties. All this requires a working memory, a mental map of where to go and what to look for. According to at least one study, the primates that hunt high and low for the quality fare tend to have larger brains than those that do not.

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