Wednesday, January 18, 2012

Genesis and Molecular Genealogy


There are five scenarios to explain the current distribution of human populations. They can be seen in the figures below.

Source: http://arslanmb.org/ArmenianDNAProject/screenshot-38.jpg


Explanation of Molecular Genealogy Models

1. Recent out of Africa: Single Origin Population model
In this model, humans are a young species that underwent a bottleneck, and Eurasians are descended from a group of Africans that migrated out of Africa. This model has been criticized for its inability to explain deep divergence in autosomal DNA (atDNA). Autosomal DNA is a term used in genetic genealogy to describe DNA which is inherited from the autosomal chromosomes.  Each set of autosome is received from your mother and your father.  Their atDNA was received from their parents and their grandparents, these becoming more disjointed with each generation since most people no longer marry close kin.  Within endogamous groups such as Abraham's Horite people, the disjunction is considerably less, and depending on the mating structure, may be preserved quite well.

In this model, the African Eve, traced by mitochondrial DNA, lived 170,000 years ago while the African Adam, traced by Y-chromosome DNA, is believed to have lived 100,000 years ago. 

The Y-chromosone determines gender and is passed virtually unchanged from father to son. The mutation rate is very slow so the Y-chromosone can be used to trace lineage back many generations. 

Mitochrondrial DNA (mtDNA) passes from mother to daughter from generation to generation with very little change and is therefore a rich source of information for those studying deep ancestry. The Horites married only Horite women, so Horite ancestry can be traced by mitochondrial DNA. As Horite rulers married half-sisters and patrilineal cousins or nieces, there would be considerabe genetic stability. This stability was valued because the Horites expected God's Son to be born of their ruler-priest caste which practiced endogamy.

Mitochondrial Eve’s line will not die out because mitochondrial DNA is passed down to both male and female offspring. On the other hand, Y-Chromosomal Adam's line will not be the same at any point in history due to the fact that male lines can die out. When this occurs, a more recent ancestor becomes the new “Adam.” The most recent common male ancestor is believed to have lived 100,000 year ago.


2. Recent out of Africa: Multiple Archaic Populations model
This model is advocated by those who believe that modern humans evolved from ancestors common to modern humans and other archaic humans (hominins), some of whom became extinct.

Dienekes Pontikos holds this view but mainains that the mating structure of the African population indicates isolated long-standing subpopulations.  In this view, Eurasians are Afro-Asiatics who descend from one of these African subpopulations. The variants that have deep origins are presumed to have evolved separately in different African subpopulations and entered the modern gene pool after unstructured mating (panmictic) became more common.

Anthropological and archaeological discoveries support the recent Out-of-Africa view for modern humans, but don't support the view of separated archaic humans who became extinct. This view lacks physical evidence that the theorized extinct hominids were unlike modern humans.  This has been shown to be false by paleontologist Jeremy DeSilva and others.

DeSilva compared the ankle joint, the tibia and the talus fossils of human ancestors between 4.12 million to 1.53 million years old, he discovered that all of the ankle joints resembled those of modern humans rather than those of apes. Chimpanzees flex their ankles 45 degrees from normal resting position. This makes it possible for apes to climb trees with great ease. While walking, humans flex their ankles a maximum of 20 degrees. The human ankle bones are quite distinct from those of apes. Read about DeSilva’s research here.

Further, the discovery of a complete fourth metatarsal of A. afarensis at Hadar that shows the deep, flat base and tarsal facets that "imply that its midfoot had no ape-like midtarsal break. These features show that the A. afarensis foot was functionally like that of modern humans." (Carol Ward, William H. Kimbel, Donald C. Johanson, Feb. 2011) Read the report here.

Multi-Regional: Recent Admixture model
This view agrees on the recent African origin of modern humans, but maintains a place for long isolated pre-existing Eurasian populations who contributed some of their mtDNA to modern humans. Multiregionalism posits that the descendants of  H. erectus evolved into Neandertals, Denisovans, and modern humans in different regions. Populations coming from Africa mingled with the other groups and modern humans emerged.

This model presumes two variants with deep common ancestry stemming from the separated Eurasian and African strains. This model has found support by analysis of the Neandertal genome. However, recent studies have revealed that only 2–3% of the genome of non-Africans might come from Neanderthals.  Further, some believe that Neandertal introgression into Eurasians can be explained by the Multiple Archaic Populations in Africa model.


Multi-Regional: Long Standing Admixture model

In this view Africa is the point of origin of human Y chromosomes and mtDNA and separation took place about 150,000 years ago. Modern humans are descended from populations from around the world between which there has been gene flow. This model explains divergence, but does not offer an explanation for the African origin of the uniparental markers, the evidence for the appearance of anatomical modernity in East Africa and the evidence for less genetic variation in Eurasia with increasing distance from East Africa. 


Ancestral Bottleneck model
This view is proposed by Blum and Jakobsson, who speculate a bottleneck 150,000 years ago in an ancestral structured population. This model combines elements of the Single Origin Population model and the Multiple Archaic Populations model. It hypothesizes a single origin model for extant humans and allows for the possibility of structure in Africa prior to the bottleneck.  The breakdown of structured mating is set before the bottleneck. 

Blum and Jakobsson write, "Through the analysis of a public DNA sequence database, we find, similar to previous estimates, that the common ancestors of autosomal and X-linked genes are indeed very old, living, on average, respectively 1,500,000 and 1,000,000 years ago. However, contrary to previous conclusions, we find that these deep gene genealogies are consistent with the Out-of-Africa scenario provided that the ancestral effective population size was approximately 14,000 individuals. We show that an ancient bottleneck in the Middle Pleistocene, possibly arising from an ancestral structured population, can reconcile the contradictory findings from the mitochondrion on the one hand, with the autosomes and the X-chromosome on the other hand."                 

In other words, they support the view that the earliest human populations were in Africa and had a mating structure that preserved a genetic identity until about 130,000 years ago. The hand axe is the most common artefact of these early East African and "Old World" peoples.


Which of these views aligns with Genesis or does Genesis suggest yet another possible model?

What do you think?

2 comments:

BibleGeorge said...

I appreciate your research and findings - this is cool stuff! I have no knowledge of molecular DNA but I'm willing to learn.
How are you getting the numerical figures such as 170,000 years?
How are you measuring years in general?
Most importantly, how would the Horites have known about Adam and Eve (and their story) without the benefit or technology of DNA testing?

BibleGeorge said...

Hi Alice,
Regarding this quote:
"DeSilva compared the ankle joint, the tibia and the talus fossils of human ancestors between 4.12 million to 1.53 million years old"

How did DeSilva get 4.12 million from looking at fossils? What is on a fossil that gives you its age?