Wet spells in the Sahara may have opened the door for early human migration. According to new evidence, water-dependent trees and shrubs grew there between 120,000 and 45,000 years ago. This suggests that changes in the weather helped early humans cross the desert on their way out of Africa.
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 Castañeda 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.
Wet spells
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 year 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.
Fossil record
Ian Tattersall of the American Museum of Natural History in New York is impressed by the findings. "They tie in approximately with the information we have from the fossil record."
Castañeda'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.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0905771106
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
Have your say
What About River Nile?
Mon Nov 09 23:12:28 GMT 2009 by Uwe Kerkow
http://mediawatchblog.org
Why make it more complicated than it is. If I had to walk from Kenia to Israel (!) I would choose the banks of river Nile. Abundant water, enough greenery and game...
I always find it fascinating what all the amateur scientist think. Uwe have you studied anthropology or any other related subject? Do you have deep knowledge in this area? If not, your comment is for naught
What About River Nile?
Tue Nov 10 08:46:21 GMT 2009 by sciencebod
http://www.colinb-sciencebuzz.blogspot.com
But there's another "amateur scientist" (amateur re human pre-history that is) who is also puzzled by this article, and seeking explanations. Yes, it would be nice once in a while to hear from the specialists, but fellow amateurs sometimes know the answer.
It's not just the Nile that provides a trans-Saharan route. It's the fact that there were permanent settlements in the north-centre of the Sahara some 7000 years ago - in the Hoggar - not just with cave paintings showing organized society (wedding ceremonies etc) but the remains of elephants, rhinos, hippos etc. That speaks of a Sahara very different from the one that exists today. If the near-centre supported humans and game 7000 years ago, imagine how much greener it may have been if one were to wind the clock back till further. That in turn suggests the existence of long-distance routes, at least for salt and ivory, and probably for a wider range of traded goods.
Even now there are a few remnant populations of Nile crocodiles living in some isolated water pools in the middle of the Sahara- all that's left of rivers that used to flow there. I thought it was well-known that the Sahara was greener about 10K ago. But that does not mean you can extrapolate backwards to assume it must have been even greener earlier on. Probably it goes up and down, and between the wet periods of 50,000 ago and 10,000 years ago, a dry period prevailed.
Okay, after voicing my amateur opinion, I had a look at Wiki:
http://en.wikipedia.org/wiki/Sahara
"The climate of the Sahara has undergone enormous variation between wet and dry over the last few hundred thousand years. During the last glacial period, the Sahara was even bigger than it is today, extending south beyond its current boundaries. The end of the glacial period brought more rain to the Sahara, from about 8000 BC to 6000 BC, perhaps due to low pressure areas over the collapsing ice sheets to the north."
That seems a little harsh. I thought it was an intelligent and interesting question. If it's based on some core misconception, I'd be interested to know what it is.
My guess (as someone who is not even an amateur scientist) is that the article does not mean the Sahara was a barrier in an absolute sense. Probably some determined individuals would have been capable of finding a route across, following rivers or waterholes. But if the area was hostile and relatively difficult to cross, it is hard to understand why large numbers of people left relatively green areas in order to cross it.
Would the author like to comment?
Your guess is right. I studied agriculture.
But please - if you call my comment naught, explain why. Thank you.
"Do you have deep knowledge in this area? If not, your comment is for naught"
If only experts on the subject with deep knowledge are allowed to voice their opinion here, the comments section will soon get pretty empty. Experts are usually busy studying their subjects after all, not writing comments on a pop science website. And where would NS gets its revenue from then?
Uwe's post was a meaningful contribution to the discussion, in contrary to yours.
Yes - I was thinking exactly that. People came out of Africa in small groups anyway. We aren't talking about mass migration.
This comment breached our terms of use and has been removed.
'And both genetic analysis and archaeology show that humans didn't spread extensively beyond Africa until 50,000 years ago' - this statement is incorrect, our line of modern-humans began migrating out of Africa around 85-80,000 years-ago via the southern point of the red sea, and had arrived in south-east Asia; china around 75.000 years-ago. 'The manju L3 (m2) has a local age of 73,000 years-ago in India', Stephen Oppenheimer, 2003; 'Out of Eden'
Whichever figure is correct, if they left by sea, were they capable of building boats or rafts? What about water, supplies, navigation etc.? We have no indication so far that they had any precursor technology. Even a documentary attempting to show how the inhabitants of Papua set out for Australia on bamboo rafts looked like wild speculation to me.
"were they capable of building boats or rafts?"
Why wouldn't they be? They were fully modern humans with the same brain capacity as us. If you ever spend time with "primitive" peoples, you'd be impressed by how ingenuously they use natural materials to craft whatever they need, and how much knowledge they have of the natural world around them. None of all that leaves fossil traces. If you doubt our ancestors were capable of building seaworthy rafts, then how do you explain the presence of Aboriginals in Australia?
As I remember now; it was the dry-spell that allowed humans to exit Africa due to falling sea-levels that allowed our walking ancestors to exit by foot alone.
Jeff Hecht is describing evidence for the wet-spell that allowed the migration of the last-migration prior to our own out of Africa; at its northern exit land-bridge around 120,000 years ago; which has been more or less static. That almost resulted in an earlier peopling of the world, if it was not for the dry spell that trapped the population in the Levant and smited them around 90,000 years ago.
There was earlier dry-spells, but modern humans did not exit Africa due to equivalent peoples on the other side, it is believed that absence of these archaic humans allowed our ancestors to finally make the historic journey
I agree with the spirit of your comment. Humans had left Eastern Africa prior to the second 'wet era' that this article describes. It may not be that the 2nd wet era allowed them to leave Eastern Africa, rather it allowed them to build up greater populations in their other settlements and also to settle in even more areas.
In other words, it is wholey possible that the wet era was not confined to Eastern Africa.
All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
If you are having a technical problem posting a comment, please contact technical support.
PRINT
SEND
