How did we come out of Africa?

by Matthew Cobb

There’s an interesting report in Science magazine today [subscription needed] about a meeting held in Gibraltar earlier this month about human evolution. Most of the debate seems to have revolved around the classification of around 28 >500,000 year old fossil hominins from Sima in Spain. Some people argue they were Homo heidelbergensis, generally viewed as an ancestor of the Neandertals.

Science reports that Ian Tattersall, from the American Museum of Natural History, argued that “two or more hominin lineages must have existed side by side in Europe for several hundred thousand years before H. sapiens arrived from Africa. One line led to the Neandertals and may have included the Sima fossils; another, rightly called H. heidelbergensis, went extinct while the Neandertals lived on until at least 30,000 years ago.”

Equally fascinating was the discussion about *how* we came out of Africa, and how many times. We seem to have originated in Africa around about 200,000 years ago, and to have left to colonize the world about 50,000 years ago. But in the “Levant” there are clear signs of co-habitation between humans and neanderthals from 75-130,000 years ago. Some people call this “the failed dispersal”. At the Gibraltar meeting, paleontologist Mike Petraglia (Oxford) summarized archeological evidence – stone tools and so on – that suggests the “failed” dispersal was not such a failure as that.

He argued that human populations may have gone on from the Levant into India and thence to Australia, where we turned up about 40,000 years ago. The archeological evidence suggests we crept along the sea-shore, at about 1km a year… If we didn’t leave the stone tools, who did? Some people at the meeting were skeptical, pointing out that archeology was not biology – without human fossils, you can’t tie the tools to us. As always with scientific arguments, evidence will decide who’s right and who’s wrong.

326_224_F3(Source: Michael Petraglia)

18 thoughts on “How did we come out of Africa?

  1. Would not the dry desert be a good place for fossil preservation? Why are there so few fossils in that area? Are there fossils of other fauna?

    1. My guess is that 1) the desert cannot support a very high population density of animals and 2) most of the time, the sediment gets eroded as often as it gets built up, which doesn’t lend itself to fossil formation.

    2. Generally speaking, for something to fossilize it has to be covered with sediment of some sort very quickly, before scavengers can get to it, and them must remain stable. I would expect most deserts would be unlikely to cover animals very quickly, especially considering the relatively uniform, featureless landscape and lack of cover make any dead organism easy pickings for airborne scavengers. Violent sandstorms would also likely spread around and break up and bones, and as Ray said the landscape is extremely unstable. In the Sahara sand dunes the size of small mountains can appear and disappear in a very short period of time. Imagine what moving that quantity of sand would do to brittle, dried-out bones.

    3. Almost all the fossils of the relevant age in the Near East and North Africa have been found in caves, not open-air sedimentary contexts.

      A very high proportion of caves open during Middle Paleolithic times have some archaeology. Archaeologists spend many years digging a cave; they are high-value targets, but few produce human remains, even those very dense in artifacts.

      Some of the work described in Balter’s article are open-air contexts in Yemen and India, these are even less likely to produce fossil humans, because they are unlikely to be sites where humans returned consistently.

      The problem is not so much that there was a low fossilization rate, but that we’re looking for things in a narrow time window. Across the entire Pleistocene, for any given 30,000 year interval in a region the size of West Asia, the modal number of fossil humans is zero.

  2. Much of the early human route from Africa to India to Australia is now under water (due to the ending of the last glacial period).

    There must be lots of stone tools under the ocean. Too bad it’s so difficult to do archaeology on the ocean floor.

    1. That depends on what one means by ‘good’.
      It is certainly compelling, at least.
      Loosely dated Rock art, and mass megafauna extinctions are, to me, the most convincing.
      Megafauna who have survived scores of climatic changes and become extinct in a matter of the blink of an eye in other better documented areas lends me to confidently state that megafaunal extinctions are the direct result of human colonization.

      1. The evidence for human occupation over 20K years in Australia is very unreliable. The mega fauna arguments are IMHO rubbish and show a clear bias as there is no credible evidence to support the connection. Rock are dating you have to be kidding. The most likely truth is some people washed up 20K years ago and then some time later multiple landings where made. The time lines and complexity of getting to Australia 50 to 60K years ago defy rational analysis

  3. matt, is that recent evidence? I find this 2003 work as well as some background here, removing some older controversial mithocondrial DNA datings and supporting the post:

    “Our study shows that humans were present at Lake Mungo as early as 50,000 to 46,000 years ago,” the authors write. “We find no evidence to support claims for human occupation or burials near 60,000 years ago.”
    “Some [scientists] argued that the previous estimate of 62,000 years for Mungo 3 was a problem for Out of Africa, but I didn’t think so,” said Chris Stringer, director of the Human Origins program at the Natural History Museum in London.

    Stringer and colleagues support the idea that at least one wave of migration out of Africa preceded the exodus that occurred 50,000 years ago, and that the earliest journeyers reached Australia.

    “Modern humans had plenty of time to reach Australia,” he said. “A rate of movement from Africa averaging only one mile a year along the coasts of southern Asia would have got people to within a boat ride of Australia in less than 10,000 years.”

    The revised dates lend additional credence to the Out of Africa model and are consistent with evidence from other sites in Australia.

    “This agrees very well with genetic data suggesting a settlement of northern Australia no earlier than 50,000 to 55,000 years ago,” said Spencer Wells, geneticist and author of the recently published book The Journey of Man: A Genetic Odyssey. Wells uses Y-chromosome data to trace the paths taken by early humans as they left Africa.

    If that has continued to stand, presumably that is the resolution. I.e. the evidence is ~ 40 – 50 ka, but 60 ka wouldn’t be a problem.

    1. D’oh! “mithocondrial” – mitochondrial.

      Oh, speaking of which, if the 60 ka figure is from the mitochondrial evidence: not that this layman profess any understanding in this, but ironically you can read John “Selective Sweeps” Hawks’ blog on why one wouldn’t want to rely on any which way mitochondrial dating evidence. IIRC as a specific point, but I assume you can reconstruct the hypothesis if you know what you are doing. [/throws up arms and lets himself out]

  4. How did we come out of Africa? I thought we were kicked out for intimating that no one of African descent would ever become president of the USA….

  5. Some people at the meeting were skeptical, pointing out that archeology was not biology – without human fossils, you can’t tie the tools to us. As always with scientific arguments, evidence will decide who’s right and who’s wrong.

    Not evidence on its own though, as we can see in this very passage. There’s also the tricky business of interpretation. New evidence, of the right kind, could decide who’s right and who’s wrong, but meanwhile competing interpretations are busy competing. This is interesting.

    (The relationship to forensics is also interesting. Found the weapon. Jolly good; but how do you know how it got there?)

  6. Some time has passed since this was written, but I am curious as to which theory holds more accuracy, the Multi-Regional or Recent African Origins?

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