SURVIVAL OF THE WEAKEST?

Evolutionary biologists have long recognised the role serendipity plays in which species thrive and which wither on the Darwinian vine. Without the asteroid impact 65 million years ago, for instance, mammals would not have spread so soon into almost every ecological niche on Earth (dinosaurs were in the way). Yet when the subject strikes as close to home as why our ancestors survived and Neanderthals did not, scientists have resisted giving chance a starring role, preferring to credit the superiority of ancient Homo sapiens.

 

 Both are descendants of Homo erectus: some spread across Eurasia beginning 1.8 million years ago and evolved into Neanderthal by 300,000 years ago, and others evolved in Africa, becoming anatomically modern by 200,000 years ago and reaching Europe some 45,000 years ago.

These arrivistes are often portrayed as technologically and culturally more advanced, with their bone and ivory (not just stone) tools and weapons, their jewellery making and cave painting—the last two evidence of symbolic thought. Finlayson has his doubts. Neanderthals may have painted, too (but on perishable surfaces); they were no slouches as toolmakers; and studies of their DNA show they had the same genes for speech that we do. "They survived for nearly 300,000 years," Clive Finlayson, director of the Gibraltar Museum,says. "That modern humans got to Australia before they penetrated Europe suggests that Neanderthals held them off for millennia. That suggests they weren't that backward." Finlayson is a noted evolutionary ecologist.

Instead, moderns were very,very lucky—so lucky that Finlayson calls what happened "survival of the weakest". About 30,000 years ago, the vast forests of Eurasia began to retreat, leaving treeless steppes and tundra and forcing forest animals to disperse over vast distances. Because they evolved in the warm climate of Africa before spreading into Europe, modern humans had a body like marathon runners, adapted to track prey over such distances. But Neanderthals were built like wrestlers.That was great for ambush hunting, which they practiced in the once ubiquitous forests, but a handicap on the steppes, where endurance mattered more.

The open, African type of terrain in which modern humans evolved their less-muscled, more-slender body type subsequently expanded so greatly in Europe.And that was "pure chance."

Thanks to recent discoveries that they were canny hunters, clever toolmakers, and probably endowed with the gift of language,Neanderthals have overcome some of the nastier calumnies hurled at them, especially that they were the "dumb brutes of the North", as evolutionary ecologist Finlayson describes their popular image.

But they have never managed to shake the charge that their extinction 30,000 years ago, when our subspecies of Homo sapiens replaced them in their European home, was their own dumb fault. Modern humans mounted a genocidal assault on them, goes one explanation, triumphing through superior skills. Moderns drove them into extinction through greater evolutionary fitness, says another, especially the moderns' greater intelligence or social advances like the sexual division of labour.

Winners get to write the textbooks. So it is no surprise that we, the children of the humans who replaced Neanderthals, "portray ourselves in the role of victors and reduce the rest (of the human lineage) to the lower echelons of vanquished," Finlayson writes. "To accept our existence as the product of chance requires a large dose of humility." But in a provocative new book, The Humans Who Went Extinct: Why Neanderthals Died Out and We Survived, he argues that chance is precisely what got us here. "A slight change of fortunes and the descendants of the Neanderthals would today be debating the demise of those other people that lived long ago," he argues.

New evidence has indicated that wet spells may have helped Stone Age humans cross the Sahara desert on their way out of Africa about 93,000 years ago.

According to a report in New Scientist, evidence indicates that water-dependent trees and shrubs grew in the Sahara desert between 120,000 and 45,000 years ago.

The Sahara would have been a formidable barrier during the Stone Age, making it hard to understand how humans made it to Europe from eastern Africa, where the earliest remains of our hominin ancestors are found.

Isla Castaneda of the Royal Netherlands Institute for Sea Research and colleagues studied land plant hydrocarbons in Saharan dust that has settled on the sea floor off west Africa over the past 192,000 years.From the ratio of carbon isotopes in the hydrocarbons, they can work out which types of plants were present at different times.

While about 40 per cent of hydrocarbons in today's dust come from water-dependent plants, this rose to 60 per cent, first between 120,000 and 110,000 ago and again from 50,000 to 45,000 years ago.So, the region seemed to be in the grip of unusually wet spells at the time.That may have been enough to allow sub-Saharan Stone Age Homo sapiens to migrate north: the first fossils of modern humans outside Africa date from 93,000 years ago in Israel.

And both genetic analysis and archaeology show that humans didn't spread extensively beyond Africa until 50,000 years ago, suggesting a second migration at the time of the second wet spell.

Castaneda's team is not the first to suggest that wet spells may have come in handy.Last year, Anne Osborne of the University of Bristol, UK, suggested that the first migrants may have used a now-buried network of river channels in the Libyan Sahara, which dates roughly 120,000 years.

Whatever it is, "we" seem to be a lucky lot.And we seem to be getting away with it!