How did modern humans get out of Africa

[RonO]

It is kind of a mystery why the Homo populations in Africa were
separated from the Homo populations in Europe and Asia for so long.
There is a recent study where they try to figure how what route modern
humans took when they finally left Africa around 60 to 80 thousand years
ago.

There are two manageable routes out of Africa. One across the Sinai
from Ethiopia into Yemen and Saudi Arabia and the other through Egypt
along the Mediterranean coast. To try to differentiate between the two
options the researchers sequenced 225 human genomes (100 from Egypt and
125 from Ethiopia).

Since these were extant human genomes they have to deal with the issue
of recent migration back into Africa. To do this they looked at
haplotypes. A chromosome is one long molecule of DNA with some
interesting sequences spread out along the strand of DNA such as genes
and their regulatory sequences. Each reproductive cycle the chromosomes
get scrambled by meiotic recombination, so that bits of your
grandparent's chromosomes that your parents inherited get mixed up and
passed down to you. This means that the chromosomal haplotype gets
chopped up and different haplotypes are formed every generation by the
recombination of different sources of the bits of chromosomes that get
put together. It also means that the bits that don't recombine and
retain the ancestral linear order along the DNA sequence get to be
shorter and shorter every generation. As amazing as it may seem when
you sequence the whole genome you can identify the very small haplotypes
that would have been present when some of our ancestors wandered out of
Africa. They can differentiate these more ancient haplotypes from the
longer haplotypes that exist due to more recent admixture as humans
migrated back to Africa within historical times.

It may not surprise anyone but Egyptians are a very mixed bag in terms
of where they came from. A lot of the Egyptian's genomes were derived
from non Africans. In recorded history, the Greeks conquered the place
and then the Romans, but most of the non African DNA comes from the
Arabic occupation of around 750 years ago.

Still, from the African haplotypes that are left they can tell that the
modern humans that made it out of Africa are more related to the
Egyptians than they are to the Ethiopians. Beats me if their analysis
is going to hold up. It seems that it wasn't just hard to get out of
Africa 60 to 80 thousand years ago, but it was difficult to go back too.

Some one is going to have to do some snooping into what the climate was
like 60,000 years ago to try to figure out why humans could not migrate
along the coast. There is a desert there today, but was it that
inhospitable?

http://www.cell.com/ajhg/abstract/S0002-9297(15)00156-1

The paper should be free to download.

Ron Okimoto

[RSNorman]

Thank you for this and for the commentary. Yes, the paper is free to
download.

Since I have no knowledge of all the anthrological background, I can
only speculate based on ignorance. As to the routes, Egypt to Sinai
vs. Ethiopia to Yemen and Arabia, I don't know how the 15 mile sea
passage across the Gulf of Aden (Bab-el-Mandel strait) would be
managed back then. The northern route through Egypt is all land,
unless the parting of the Red Sea can be back dated some 70 thousand
years.

And as to the reverse route, once vast new habitats were opened up
with good environmental conditions, why return through difficult and
perilous regions? Something impelled the outward expansion but
pressure to move on, or even a drunkard's walk, is extremely unlikely
to retrace a path. Mathematically, the probability of a random walk
returning to the origin in 2 dimensions may be 1 but the expected time
it takes is, I believe, infinite. So realistically it doesn't happen
in any reasonable time frame.

[jillery]

On Sat, 30 May 2015 08:10:34 -0500, RonO <roki...@cox.net> wrote:

Here's a thought. Even if it was as hard physically to leave Africa
as it was to return, the motivation to return might have been less
than the motivation to leave. Why go back to a dry overcrowded Africa
when you have the cool and fertile entirety of the Asian shores and
plains just waiting for you?

--
Intelligence is never insulting.

[passer...@gmail.com]

Humans have been coming and going from Africa for millions of years. No 1000' walls separating Africa from the rest of the humans on the planet.

[Mr. B1ack]

Um ... they *walked* dude ....... easy. Anybody
can do it.

[RonO]

I would suspect that the Nile delta region around Cairo was one of the
most heavily populated places on earth for humans during the last 80,000
years unless there was some disease issue such as malaria. The strange
thing is that there is only a couple hundred miles of desert between the
delta and Israel where Neandertals are known to have lived. When you
look at it on a map it seems strange that the two populations could have
been almost touching in that part of the world and yet remain separate.

I would think that population pressure would be constantly pushing
people out of the delta, so it would have been more difficult to go back
and reestablish residence than it would be to leave.

Homo erectus made it out to Indonesia and even with the drop in sea
levels during the ice ages they would have had to be decent mariners.
My guess is that humans could have made it across the straits, and some
likely did, but they did not do so in enough numbers to either establish
a viable population or just could not make it for some reason such as
lack of fresh water.

Ron Okimoto

[RonO]

I do not know what the climate was like in or out of Africa at that
time. It was during the last glacial period 10,000 to 120,000 years
ago, and the glaciers supposedly didn't max out until around 20,000
years ago. My guess is that climate was more temperate because the
jungle fowl that were left along the coastal islands have a long day
breeding cycle (they breed in the spring when the weather is usually
warming up). More tropical birds often breed with the rain cycle that
is they are often short day breeders. So they may have been feeling the
winters, and it may have been more pleasant in the summer than the
climate is today in South Asia.

Ron Okimoto

[s...@broadinstitute.org]

I've only read the abstract, but there's a very large assumption there: the current genetic composition of the two places in any way reflects its composition 60,000 years ago. One thing that's become clear from the study of ancient DNA, especially in Europe, is that wholesale movement and replacement of populations have occurred repeatedly over that timespan.

[passer...@gmail.com]

Neanderthals were sailing around the Med. 100,000 years ago. I mean, if some think they and "moderns" couldn't walk. They populated all those islands.

And those caves in Israel had "Neanderthals" and "moderns" alternately back and forth and it's darn hard to tell the difference when you look.

"IT LOOKS like Neanderthals may have beaten modern humans to the seas. Growing evidence suggests our extinct cousins criss-crossed the Mediterranean in boats from 100,000 years ago..."
http://www.newscientist.com/article/mg21328544.800-neanderthals-were-ancient-mariners.html#.VWslGvldV8E

One big interbreeding species since long before Homo Erectus.

[s...@broadinstitute.org]

On Saturday, May 30, 2015 at 9:13:07 AM UTC-4, Ron O wrote:

See more wide-ranging and detailed criticism of this paper here: http://www.unz.com/gnxp/modern-genetic-variation-is-poor-representation-of-past-genetic-variation

and here (allegedly from one of the reviewers of the paper):
https://pubpeer.com/publications/2059A0004FC74E46D8E44423B7BD34#fb30964

[RonO]

As I note above only time and more analysis will tell if their
conclusions hold up. The paper claims to have dealt with the issue of
back migration, and they claim that they could identify it if it had
occurred. For their analysis to hold up they would have had to exclude
more ancient back migration and as I note they claim that this is what
they did.

Someone else may come up with a way to tell if they did or not.

Ron Okimoto