Expansion of
Man
(after Ambrose, J. Hum. Evol. 34, 623-651)
Oct. 5th, 2000
E&PS 106
One of the most convincing concepts for the origin of Modern Man is the African Origin model. Evidence includes:
· Fossils
· Archaeology
· paleobiogeography
· paleoclimatology
· genetic structure in extant humans
Three models are proposed:
1. Regional Continuity or Multiregional Evolution Model
2. Replacement
3. Weak Garden of Eden
4. Multiple Dispersals
The Problem:
The time that a species has been isolated can be estimated from it’s genetic differences. The frequency of any gene in a population is an indication of its selective value, and gene frequency rests on certain genetic factors. Most organisms, for example, carry two genes, called alleles, for any trait. For some things, the number of alleles responsible for a trait are much more. Blood, for example, has 8 different alleles. Sometimes one allele dominates the other. The dominating allele is called the heterozygous dominant; the allele being dominated is called the heterozygous recessive. (This is like the red pea flower of Mendel). If an identical pair of alleles both of which are either recessive or dominant occurs on a chromosome, the alleles are called homozygotes because neither one of the pair dominates the other. A hidden pool of variation lies in a population's alleles. The number of differences of paired alleles gives us some idea of how long a population has been isolated. For humans, the time of isolation is only on the order of 100,000 years. So any theory for their spread around the world (exclusive of the Americas) must take this into account.
1) Regional Continuity or Hybridization model
Homo Erectus left Africa between around 1.8 to 1.0 m.y.a. Distinct regions were populated and evolved into modern Man. Gene flow between regions prevented speciation and carried all regional populations across the threshold of modern humanity in close synchrony, while allowing for local variations to evolve. There should be some intra-regionally distinctive alleles, for example, if Neanderthals contributed to the gene pool of western Eurasians. But no non-African alleles have yet been found. In addition, no significant morphological distinctions are seen in modern humans. Neanderthal is quite different, but there is no evidence of it in modern humans.
2) Replacement Model
a) Out of Africa in the lower Pleistocene. Sometime during the last 140,000 to 100,000 years, modern man came out of Africa and replaced earlier Man in the rest of the Old World. Earlier “Archaic” Homo sapiens, including Neanderthal, had no genetic contribution and were wiped out by the more advanced African species. This is supported by several lines of evidence:
1. Modern Homo sapiens fossils appeared in Africa up to 250,000 y.b.p., long before they appear elsewhere in the world. The oldest modern fossils outside of Africa are at ‘Africa’s doorstep’ in the Levant[1] at 100,000 y.b.p.
This was a warm period (from ice cores, we know that ~130-~74 ka was warm), so that it is reasonable that they would migrate at this time.
2. The total genetic diversity of humans is extremely low, but highest in Africa. Compared to Great Apes, there is an extremely low level of within- and between-population genetic variations in humans.
3. The mtDNA (mitochondrial DNA) of Africans has much greater diversity than European and Asian populations – about double. Geneticists trace back the non-African populations to an “African Eve” that lived some 140-200 thousand years before present (ka).
b) Replacement models with bottleneck –
A bottleneck is a time of significantly decreased population. The gene pool goes way down. Bottlenecks can be identified by the paired
differences of nucleotide sequences. If
there has been very little time for mutation and evolution to occur, the
differences are very small. The pattern
of differences can be used to identify bottlenecks – times when the population
was very small. Time and size have
inverse effect. A small bottleneck long
ago is equivalent to a lesser bottleneck more recently.
How small might the bottleneck have been? As small as 40 breeding females for 200 years or 4000 over a period of 20,000 years. Reasonable estimates are several thousand.
(Bottlenecks are seen in other species. Cheetahs have undergone an extreme bottleneck, presumably when conditions were warmer and savannah was replaced by jungles (cheetahs like savannahs). Modern bison have an extremely low diversity (even before they were nearly wiped out some 100 years ago). It has been postulated that a small population came into N. America only ~10,000 years ago.)
c) Weak Garden of Eden with bottleneck – Dispersion
at about 100 ka into distinct regions, but without substantial growth. There is some genetic diversity that occurs,
but this is wiped out by a bottleneck at 50 ka.
Now the question arises: What caused the bottlenecks?
There are several possibilities:
1. Widespread disease. Possible, but as of now untestable.
2. Global climate change. Between 74 and 60 ka, the Earth was going through an ice age. This could have reduced human population size. In fact, at this time, Neanderthal was replacing modern man in the Levant, supporting their better adaptability to cold climate.
3. Volcanic winter. A cataclysmic volcano? This is possible and there is a candidate. At 74 ka, the Toba eruption occurred. It is the second largest eruption known. Compare:
The largest historical eruption is
Tambora 20 km3 of ash
Mt. St. Helens 0.2 km3 of ash
Toba 800 km3 of ash
Toba covered India and changed the climate of
the world for 6 years. 6 years of volcanic
winter followed by 1000 years of the coldest, driest climate of the Late
Quaternary would have had a significant impact on human population. This is known from ice records.
The world population probably has looked something like this.
North America
For a long time, all indications were that Man came to the new World as big game hunters across the Bering land bridge about 15,000 y.b.p. created by lowering of sea level. They were bottled up in Alaska for several thousand years and then poured into the Americas starting about 12,000 y.b.p.. These are called Clovis Man, after the site in Clovis, NM where they were first seen.
There have been a number of earlier sites described, but they have all been discredited or ignored. *It isn’t possible, right?*.
Now, in Monte Verde, Chile, Archaeologists have a 12,500 year old site. Everyone accepts this. It means pre Clovis. And there is evidence for population as old as 30,000 years.
How could they have gotten there? Boat. Boating from the Kuril Islands and other Asian localities. These are known to be at least 20,000 y.b.p. There is even postulation that they came from Europe, but this is not established other than by circumstantial DNA evidence.
Why no record? If they left no tools – not big game hunters – they would have little evidence. Also, if they were by the sea, sea level rises would obliterate their existence. Also, since the last ice age, there has been lots of erosion, so evidence is gone.
DNA evidence. DNA was isolated 15-30 ka. Coming from Siberia and Mongolia.
Language evidence. 140 different languages. It takes about 6,000 y of isolation for a new language. Johanna Nichols, a language professor, says it would take at least 35 ky for such divergence to occur. “To hell with the archaeological evidence”.
Verbal evidence? – In Queen Charlotte Island, Canada, Daryl Fedje found a spear point far out in the water. How could it have gotten there?
Tribal lore of the present-day Haida nation includes tales of times ‘when the islands were far larger and surrounded by grassy plains, and of subsequent, fast-rising oceans when a supernatural ‘flood tide woman’ forced the Haida to move their villages to higher ground.