Out of Africa - migration size

[arlesterc]

I have had a hard time Googling to find out some specifics about the Out of Africa migration theories - here are the questions I have not been able to determine - and maybe they are not determinable right now - I understand that possibility.

 

So, what is meant by a wave - a single bunch of people moving out of Africa as a single bunch one shot move or several bunches moving over a short period of time?

 

How many people would be in such bunches?

 

Can these bunches who left be traced to a specific fairly pinpointed place of leaving/specific cohesive group or did they come from different places maybe unknown to each other? Africa, East Africa, Northeast Africa, Kenya, etc. are fairly 'big' locations - can the origin be traced a little more finely.

 

With either scenario is the assumption then the moving stopped completely - no more bunch/bunches/individuals moving out of Africa and if so why would that have been the case? Why would it stop?

 

As far as the moving itself would it have been a constant moving so trekking day after day after day and finally finding the special spot and settling in?

 

Or would it have been the bunch breaking into smaller bunches settling at different points some closer, some farther away from the origin?

 

Any enlightenment is appreciated.

[CharonY]

The "wave" is not a specific population walking toward a goal, rather it refers to overall migration within a time span and used to distinguish it from other major migration pattern. It is probably more accurate to thing in terms of expansion or dispersal that went on for ten thousands of years. Nor was the push unidirectional, but there are patterns of back and forth.

[Strange]

This might be of interest:

"DNA hints at earlier human exodus from Africa"

http://www.bbc.co.uk/news/science-environment-37408014

[arlesterc]

Thanks. A little more digging and I found this:

 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2953791/

The migration of modern humans out of Africa is thought to be accompanied by a population bottleneck. The size of the population(s) migrating out of Africa is estimated to be ~600 effective founding females (i.e., census size of ~1800 females) on the basis of mtDNA evidence (62, 120), to be ~1000 effective founding males and females (i.e., census size of ~3000 individuals) based on the analysis of 783 autosomal microsatellites genotyped in the Center d’Etude du Polymorphisme Humain (CEPH) human genome diversity panel (HGDP) (112), and to be ~1500 (i.e., a census size of ~4500 individuals) based on a combined analysis of mtDNA, Y chromosome, and X chromosome nucleotide diversity data (72). These estimates imply that Eurasians must have rapidly expanded to a larger size to account for estimates of a long-term effective population size (N_e) of ~10,000 individuals (census size of ~30,000 individuals) for global populations (172, 243). Indeed, several recent studies indicate a rapid expansion of Eurasian populations within the past ~50,000 years, whereas Africans have maintained a large effective population size (72, 125, 243).

 

So this 'wave of the first homo sapiens out of Africa' if it is correct seems to indicate a one-time relatively small total number. It however doesn't say whether they moved out en masse, how long a period was involved, incremental structure of move, relations between the members of the groups - families, clans, neigbours - where from, etc.

 

Is there any reason to believe this number/theory is not accurate? Any place to find out more about these few thousand people?

[CharonY]

We are talking about slightly different but related aspects here. Bottleneck population are formed when a subsequent population orginates from a subset of the original population. It does not mean that a part just went on and got isolated from the rest (though it could have happened), but for human migration the more likely scenario (given the long time of ca. 80k years). Or in other words, the populations that established and could be traced to current humans is the aforementioned sized. It does not mean that there haven't been more, but it does mean that they have not left descendants.

I am not up to date about what the current consensus but there were basically two main models. The one is that a singular event occurred in which all non-Africans trace their origin to a single population 40-80k years ago, the second suggests that initial migration may have started 120-130k years with subsequent migration waves, which I assume what the original OP was asking.

 

While there are some newer studies based on modeling support the latter hypothesis, others using genomic information

However, a series of articles, based on genomic analyses rather than the traditional use of select genetic markers found evidence that may or may not be supportive of either.

Pagani et al (Nature 2016) seem to suggest that an early expansion occured out of Africa to Australasia some 120k years as the ancestry from Papua New Guineans seem to have separated earlier from the African population than other Eurasians. Others have suggested a singular population that then split out into two waves, one eventually ending up in Australia and the others becoming the ancestors of the Eurasians. Here the "waves" if you will would refer to the split and direction of the original population(s). And again, it does not mean that the population under consideration has to be one given generation (such as parent child and grandparents) but could consist of several generations over a given time frame. After all, we are only able to look at relative contributions to the gene pool with a relatively low resolution. And an added issue is that of course populations that do not leave descendants to our days would not be easily (or at all) traced. Ancient DNA (rather than DNA from current populations) have shown that this has happened quite a bit (a lot of data surrounding the Neanderthal DNA in the last few years have discussed that).

 

And obviously both hypothesis could be in agreement with data if one e.g. assumes that no intermixing of the original population and the later migration waves have occurred.

 

Edit: regarding the specifics of quantifying a population using population genetic approaches I would defer to Arete. There are various approaches and models being utilized, with varying sensitivity to the input data and I lack the expertise to critique the respective approaches.

Edited September 23, 2016 by CharonY

[arlesterc]

Thanks for the additional info and detail.

 

However I am still not getting a picture (and perhaps there is no picture to be had) of the 'flow' out of Africa. At one extreme there would be a single small group of homo sapien hunter-gatherers who moved out of Africa in one 'voyage' and from that group came the poplulations of the world outside of Africa. At the other end of the spectrum it may be let's say 10000 small small groups of homo sapien hunter-gatherers who trekked out at varying times over a period of 10000 years. (Again these are just broad-brush concepts.) What I would like to know is an approximate range of the number of groups/total size of the 'migrating' groups and the approximate timeline of the moving. So for instance - 500 - 1000 small groups of hunter gatherers for a total population of 3000-6000 individuals coming out of Africa over a period of 500 - 1000 years. So whether two waves or one wave or three waves what did that wave look like in numbers and time. It seems to me there has to be some guesses have been made - I just can't readily locate them other than the obscure article I referred to. I am not interested in the dynamics after a group has made it out of Africa. I am interested only in how many got to Plymouth Rock so to speak. Whether they returned, parted after Plymouth rock, is not of interest to me at this stage. I am just trying to get a guesstimate of how many came out of Africa and over how long a period of time that took place - again whichever wave theory is involved. If it's the case there were bursts of migrants during certain periods but inbetween there was continuous migration albeit very small and scattered that would be of value for me to know as well. If there are some population specialists who could provide theories as to how many 'Out of Africa' individuals would have been necessary to get to the Out of Africa population we have today working with the various timelines of the different wave theories that are in play now that would be helpful. For instance, if the wave is assumed ot be 50 K - 70 K ago then it would have taken x - y original population to get to the population we have now. However if there were two waves and one was 80-60 K ago and then other was 30-50 K ago then the populations of each would have been w-x and y-z respectively to get to the population we have today. I understand that these might be conjectures/speculation more than theory but still would like to see what some experts might conjecture at this point in time.

 

Again I appreciate the ongoing attention to my query and all feedback/pointers received.

[CharonY]

Afaik, we do not have the tools to get that detailed info from that time.