I need some help from any scientific sages that visit here.
One of my favourite Darwinian myths is the Great Trek out of Africa, which holds that homo sapiens originated in East Africa and then “burst out” (cf. Ernst Mayer) for reasons unknown and started trekking to the four corners of the globe. Now, granted myth is not a synonym for error, but my layman’s impression is that, for a gang that is always prattling on about the importance of objective evidence, they hold to this truly fantastic tale on the flimsiest of probative historical records and in the face of some pretty profound questions about how and why. Nobody seems to know how it happened and there appears to be little fossil evidence of a mass population spread. In other words, they seem to be shooting blind. I suspect their firm belief has more to do with Paley than Darwin in the sense that they have no other cogent explanation and can’t conceive of any alternative–just as a watch needs a watchmaker, so a dispersed species needs a voyage. Yet presumably this is being passed on to students and the general population as if it were as irrefutable as germ theory.
This week I have been trying to track two stories here and here that appear to encapsulate the two competing versions. One states that tens of thousands of years ago man emerged from East Africa and started spreading hither and yon while retaining his species “purity”. The other seems to suggest the same geographical origin, but that the whole thing is complicated by the fact that we mated with some of the foxier Neanderthals who were already hitch-hiking the globe. The period we are talking about seems to cover upwards of fifty thousand years. Colour me confused, but am I right in concluding that the evidence for both versions consists of (count ‘em) one skull? Are we really just two skulls ahead of Genecis?
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Well, a bit more than that, but it is certainly a strongly debated subject in the field due to the relatively paltry amount of evidence.
It comes down to fossil ages. We have found (by far) the oldest fossils in Africa. Based on this, the natural presumption is that mankind originated there. Given that all humans can interbreed, there is a further presumption for spreading rather than independent evolution. There's also the mitochondrial "Eve", which indicates a single point of origin. The "out of Africa" scenario is therefore the default theory (I have never seen claims of "mass population transfers", but a spread the way clover spreads in your yard).
One can raise many substantive objections to this theory. For instance, are the fossil ages simply the result of selection bias, i.e. we have looked more in Africa? Could modern humans be a hybrid of multiple, independently evolved proto-humans? The Neandertal inter-breeding theory is a variant of this one. Note that hybridization theories don't preclude the existence of an Eve in one of the source species.
Cro-magnon man, our presumed ancestor, is a bit mysterious as well. He seems to just show up, rather suddenly, and take over (a common sci-fi trope is that Cro-magnon is alien or alien created, to explain his sudden appearance).
So, if it all got turned around next week, I wouldn't be much surprised.
I don't think that selection bias is a high-probability explanation, since although we've done more formal digging in Africa, Europe has been much more dug up for personal and commercial reasons.
Unless we want to theorize that all of the million-year-old European skulls got dug up, smashed, and scattered long ago, which is certainly possible, then by far the most likely thing is that they never existed.
What about Asia? And there are other forms of selection bias, such as climate (i.e., dry desert vs. wet forests).
Yes, it's a case of a gradual spread rather than a mass exodus.
I'm not sure what the alternative is. I don't see how either Creationists or Darwinists can accept two separate acts of creation/evolution.
(Sorry - my first attempt had so many typos it was unreadable.)
First, there are still human beings in Africa.
Second, the populating of the globe by humans is not a specifically darwinism problem.
Even if you believe in the Genesis version of Adam and Eve you have to explain how humans got everywhere.
I'm not sure why you think in terms of 'expulsion', as if we assume that all the humans up and left Africa.
The right word has just occured to me at last: 'expansion' from Eden - not expulsion.
The Out of Africa thesis is based on, among other things, the fact that Mitochondrial Eve lived in Africa.
But it is controversial. In the 90s it was highly disputed but it seems to be back as the best current theory.
(Note that Mitochondrial Eve was not the only female homo sapiens on the planet at the time, nor the first female homo sapiens, and she lived tens of thousands of years before Y Chromosonal Adam. She's just the most recent common matrilineal ancestor of each of the 6 billion odd human beings alive on the planet today.)
This is a highly readable layman's summary of the subject.
Note that Mitochondrial Eve was not the only female homo sapiens on the planet at the time,
Neither was the other Eve.
Brit, why do you lose those well-honed senses of irony and poetry that won you so many admirers when you tackle this subject?
Oh nice, I'm losing points for style now.
I thought Expansion from Eden was rather good. I hope I coined it.
There is also the fact that the level of human genetic diversity is higher in Africa than any other continent, implying a longer occupation by humans.
Peter:
I doubt there is any significant difference between the "out of Africa" theory, and the virtually indisputable "out of the Bering Straits" theory.
If you have no argument with the latter, why does the former pose a problem?
Mr. Burnet;
I think your problem is that you have only two values for theories, wrong and fully validated. It's not a court of law, and our current theory the clear cut at the end of a trial. It's far more like theories of capabilities in the foreign intelligence area.
'Neither was the other Eve.'
That's not Catholic teaching.
As for the original question:
The conclusion that the birds of the Galapagos derive from birds in western S. America is based on no skulls. There are lots of different lines of evidence in biogeography.
We know a few things. There were people in Africa 50K years ago and in Australia 40K years ago.
Cavalli-Sforza et al. can trace the migration of the Africans along the littoral of southern Asia by the gradient of mutations (always decreasing) in male mitochronrial DNA of the people who still live at the waypoints.
(This kind of evidence is fast disappearing. You could now, as Orrin likes to point out, find Somali mutations in Maine, but you could not find a gradient at the waypoints.)
The advance works out to a few miles a decade.
Curiously, Africans did not reach Spain by crossing the Strait of Gibraltar, although it is less of an obstacle, you would think, than several of the straits (notably Lombok) that the Africans had to cross to get to Australia.
The DNA gradients show that modern humans migrated up through China and got to Europe from the east and very late compared to most other places.
A good introduction is 'The Journey of Man' by Spencer Wells, and you can read my review of that at Amazon.com
Peter:
But the notion that everybody then spread out (i.e. walked) more or less uni-directionally all over the globe picking up all manner of different racial characteristics along the way for reasons no one can state authoritatively is one that really should have quite a bit more evidence in support before we all bet our intellectual mortgages on it, no?
I'll go with "no".
Within the realm of relatively undebatable fact (excluding Young Earth Creationists) goes:
-- there were no humans in North America until about 13,000 years ago.
-- the entire indigenous N. American population came via one or several migrations across the Bering Straits
-- the intervening 13,000 years was sufficient time to change a homogenous founder population into a N. American population with characteristics ranging from Eskimos to Plains Indians to Tierra del Fuegans.
Interestingly, that advance works out to be a few miles per decade.
As far as unexplained racial characteristics goes, they are entirely explained (that is, the fact they exist, as opposed to reasons for their existence).
Small populations, such as these, can have alleles disappear, or predominate, within isolated populations in one generation, simply by chance.
Harry might have added that the genetic variation of eyebrow mites completely parallels human migration.
The Steve Olson book I linkid to above is brilliant on the genetic reasons for 'racial' characteristics - and on why race is fundamentally an illusion.
M Ali:
You're not going to see any English caber-tossing champions or American cricketing geniuses either.
The illusion lies in saying "this person is that race, and his ancesters and descendents will be the same race."
Race boundaries are impossible to draw. Different people have tried to divide homo sapiens into three races, five races, thirty races, sometimes thousands of 'microraces'. Humans just don't fit neatly into distinct packages. The lines blur far too much, we are all a genetic mish-mash and all exactly as distant from mitochondrial Eve as every one else.
Any homo sapiens can mate with any other of the opposite sex. All 'race' reflects is that people tend to choose their partners locally.
Which is to say, 'race' is a classic mistake of Platonic thinking.
Yes, you can tell the difference between an average Norweigan and an average Australian aboriginie.
But there are no Races, of which any particular individual is a more or less imperfect member.
If there are races, what are they, and how do you define them?
No West African power lifters? You should meet Herschel Walker.
I don't know for sure his ancestry, but it is mostly African and, statistically, likely to be West African.
I used to attend the Drake Relays every year and sit on the rail for the 100-m dash. walker, who won the Heismann Trophy in American football, was the most muscular athlete I ever encountered (and I used to be a sports reporter).
It was funny to see him matching stride for stride sprinters half his size.
As for the dispersion of human groups, a few points.
The home group does not move very far -- that's why Cavalli-Sforza can find distinct DNA profiles a few miles from each other. But the excess population moves on, to create new stationary groups.
It is rather like a plant reproducing by means of stolons.
The Inuit have not been in the high arctice for thousands of years. At least, not many thousands. It is thought they have been there a relatively short time.
Linguistic analysis can help here. The Navajos and Apaches speak a language who closest relatives are thousands of miles away in Canada.
The mutations that human groups display are showy (except when they are not), but trivial. They do not, for example, ever rise to the level of creating behavioral isolators to reproduction across groups.
In, eg, birds and fruit flies, genetic differences that are, superficially, no more dramatic do create reproductive isolation and thus speciation.
-- It is rather like a plant reproducing by means of stolons. --
You mean like clover?
One could argue, in this vein, that genetic drift for humans occurred primarily on the edges of the expansion and once that stopped 10..15 k-years ago, so did most of the drift.
Inspired by Skipper, I have done a bit of research into just when people got to the Americas, and it appears to be a lot earlier than we're talking about here.
If the 50,000-years-ago evidence holds up, Katie bar the door.
It's not going to hold up. There's a dog that didn't bark in the night problem there.
Where are the intermediate fossils (or hearths, which would be just a valuable)?
Harry:
Whether it does or not, I'd rather be in the position of having to adjust my theory to fit the facts, rather than the other way around.
The absence of intermediate evidence is as well established a fact as any in New World archaeology.
It might turn out to be a wrong fact, but the chances are not good. They've already looked.
Nope, unless you are willing to define 'race' down to meaninglessness.
'Genetic distance' does not equal 'functional distance.' The whole point of the exercise is to track mutations that don't do anything (as far as we can tell).
We can easily distinguish, say, hair color, and if we add in texture etc. and some experience of different places, I bet most of us could identify isolated scalps to different locales.
But is anybody claiming that hair color equates to the kind of functional difference that is the reason for being of racism/racialism?
That's why I mentioned Walker. He could have been a leading power lifter if it paid like football.
If you are going to use sprinting as an example of the reality of racial differences, there is a flip side to the coin that has Jesse Owens humiliating the Germans at the Olympics -- Armin Hary.
That makes for a strong statement in favor of a weak racism. Not sure where that gets us.
There's no question that some 'families' can do what other families are incapable. The endurance of people living in very high mountains, for example.
This is the sort of thing that, selected for long enough, probably would result in a biologically meaningful race. But I see no evidence that it has gotten that far yet; or even a solid proof that the differences are mostly genetic.
And of particular traits, that's the one I know of that seems most clearly established and defined.
M Ali:
All you are doing is pointing out the obvious truism that some people with certain physical characteristics are better suited to things where those characteristics are an advantage.
It says nothing about 'race'. Race is fundamentally a Platonic concept. It implies that there are a certain number of Races independently existent, and individuals belong to them or they don't.
But genetics shows this is nonsense. It is literally impossible to categorise humans like that.
The general physical differences you see between average members of different populations (the most obvious being skin pigmentation, eye shape and height) are genetically negligible and only exist because people have mostly mated with people who live nearby, because of local cultural ideas of sexual attractiveness, and very distantly and not in all cases, localised environmental benefits.
'weak racism' was ill chosen. I should have said 'a weak concept of human races'
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