A question on the genome

[GelJipper]

Hello everyone,

I was recently reading about a near human extinction theory potentially involving a volcanic eruption around 70 thousand years ago. Just thinking about this led me to wonder how time and population is understood through genetics.

Also, how can a number/population size of ancestors be determined based on our genes? How could someone predict that when this near extinction occurred, there were 10,000 people remaining, or 30 thousand, or 100 thousand etc.? And how, with use of genetics can time be determined? Why guess that these events occurred 70,000 years ago, why not a million years ago or 40,000?

I am interested in how people understand these things, purely from a biologic stance with use of our DNA. I am not interested in related evidence from geology or paleontology.

Thanks everyone,

[relaxin]

Where did you find this theory of near human extinction? Homo sapiens migrated out of Africa about 125,000 years ago and reached Asia about 75,000 years ago. If there were major volcanic eruptions at 70,000 years ago, it would not be a global disaster. Some pockets of Homo sapiens would have survived.

Global flood would have wiped out human race, if Noah did not build the Ark.

[GelJipper]

Google "Toba catastrophe theory".

I am not so much interested in the theory as i am in the practices used that allow people to determine population size and the time in which population size fluctuates (strictly with use of our genome).

[relaxin]

Oh yes, that theory is based on a series of big explosion of Mount Toba on the Indonesian island of Sumatra about 75,000 years ago. No matter how big the eruptions were, they should not affect the population already moved on to Europe and Far East.

I am wondering how you can construct the changes in human populations so far back in the past on the basis of human genome. The human genome is "contaminated" with Neanderthal and Denisovan DNA through interbreeding. Can you tell if the interbreeding occurred before or after the Toba explosion?

[GelJipper]

For relaxin,

I am the person asking the questions to you guys. I am not sure if your questions are directed at me, but i am not educated in this field to answer them.

[relaxin]

I am not working in the field of human migration based on genomic sequence distribution around the globe either. But I am also interested on how this is done. So I throw out some questions in the hope that some experts in this field can explain this to me.

Yes, the Toba eruptions occurred about 75,000 years ago. But I doubt these reduced the human gene pool to a limited size, as many have already migrated to Europe and Far East.

[mchlbrmn]

This also is not my field. I think that ''they" can apply statistical methods to the genome do deduce a lot. These methods are new, and as more data and analysis techniques become available, I've seen results that were disputed.
An example, I think, is that small population sizes tend to become less diverse. In the distribution of genes each parent randomly gives each child a gene allele from either one or the other of the grandparents. In a small population, an infrequent gene can be lost. Larger populations tend to stay more diverse, because a gene must be represented at a far lower percentage to actually be lost.
I'm not sure how large a population this can be applied to for the analysis.
I have read that homo sapiens have gone through several population bottle necks and are genetically far less diverse than most other primate species.

[relaxin]

I guess the human gene pool size has been distorted by one man, Genghis Khan; about 0.5% of men in the world carry his Y-chromosome.

[bonigohicu]

Actually what you guys trying to prove here? I didn't understand anything.

[relaxin]

Don't worry, I do not understand much either. We are told the human race started with Adam and Eve, and then God killed everyone except for members of Noah's family. On the other hand, human migrated from Africa to Europe and Asia, interbred with Neanderthals and Denisovans on the way. So human gene pool is very small, it looks big because of contamination of Neanderthal and Denisovan DNA, but it also looks small because of Genghis Khan's DNA.

[Carolyn710]

The proposal that all mitochondrial DNA (mtDNA) types in contemporary humans stem from a common ancestor present in an African population some 200,000 years ago has attracted much attention. To study this proposal further, two hypervariable segments of mtDNA were sequenced from 189 people of diverse geographic origin, including 121 native Africans. Geographic specificity was observed in that identical mtDNA types are shared within but not between populations. A tree relating these mtDNA sequences to one another and to a chimpanzee sequence has many deep branches leading exclusively to African mtDNAs. An African origin for human mtDNA is supported by two statistical tests. With the use of the chimpanzee and human sequences to calibrate the rate of mtDNA evolution, the age of the common human mtDNA ancestor is placed between 166,000 and 249,000 years. These results thus support and extend the African origin hypothesis of human mtDNA evolution.