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thistle
Guest
I’m glad you explained that so simply!! lol
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hecd2
Guest
Tee hee! You’re right. OK - for those who had trouble with the description above let me try a simpler explanation for one element of it.
Many genes occur within a species in slightly different forms. Genes that have more tham one variant within a species are called polymorphic. Each different version of the gene is called an allele.
Some genes occur in a species in only one variant or allele. Some genes occur with only a small number of variants or alleles. Other genes occur in many different variants - in that case the gene is said to be highly polymorphic.
Now then, a healthy normal human couple can carry, at the very most, four different alleles for one gene. That’s because there are two copies of each gene in each individual - one inherited from their father and one from their mother. So in two people there can be no more than four alleles. However in many genes there are vastly more than four alleles. On the DRB1 gene for example there were 58 human alleles known in 1995 when Ayala did his analysis. Since then many more have been found so that by 1998 Bergstrom et al, *Recent Origin of DRB1 alleles and implications for human evolution’, *Nature Genetics 18, 237 (1998) was reporting that there are 135 alleles on the locus, and in 2004, Screuder et al in *The HLA Dictionary, *Tissue Antigens 65, 1 - 55 reported 297 alleles of DRB1 that had been discovered.
Now, you might well say that all the additional alleles in the human population are the result of mutations occurring in the human lineage since Adam and Eve passed on their maximum of four alleles at each locus to their offspring. But that doesn’t hold water. The alleles aren’t just uniformly different. We can gather them together into groups or families of alleles. Alleles in different groups are different from alleles in other groups at many nucleotides (a nucleotide is a like a letter in the DNA code). Within a group, alleles are quite similar, differing perhaps by just one letter from another allele in the same group. In other words, genes mutate and diverge and form families just like organisms. Groups of alleles that are very different are likely to have diverged a long time ago - more recent new alleles are very similar to their siblings in the same group.
Not only that, but many of the major groups of allele on the DRB1 gene are shared with our closest living relatives the chimpanzee, and other great apes. By comparing the groups of alleles in humans and the groups of alleles in chimpanzees, we can determine a date for when the different groups diverged from one another. If a group is present in both species its origin must have pre-dated the divergence of the chimp and human lineages. Many of the DRB1 groups of alleles diverged many millions of years in the past, before the divergence of the human chimpanzee lineages. Ayala calculates that six million years ago at the time of the chimp-human divergence there were 21 lineages of allele which have survived to today, and that is not possible if humans have passed through a population bottleneck of just one couple.
Such a severe bottleneck would leave an unmistakeable signature on other parts of the genome too - other polymorphic sites in the autosomes (the non-sex chromosomes); on the Y-chromosome; in the mitochondrial DNA. All of these analyses agree that the minimum human lineage bottleneck in the last six million was 10,000 individuals and that a bottleneck of two people in the last 200,000 years is just not tenable.
Is that better? I don’t think I can make it simpler without misrepresenting it.
Alec
evolutionpages.com
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Pensees
Guest
Does this not assume that mEve was descended from earlier forms? Is it only taken for granted that mEve had a mother?
“But if we shared common ancestry in Africa only 200,000 years ago, then these older Homo erectus populations in Europe and Asia are not ancestral to Homo sapiens, and we evolved from a later branching event in Africa.”
Steven Jay Gould, geocities.com/Wellesley/7261/eve.html
Homo heidelbergensis
“H. heidelbergensis (Heidelberg Man) lived from about 800 thousand years ago (TYA) to about 300 TYA. Also proposed as Homo sapiens heidelbergensis and Homo sapiens paleohungaricus.”
en.wikipedia.org/wiki/Human_evolution#Homo_heidelbergensis
Homo antecessor
“The combined US-ESR age estimates indicate human occupation from about 800,00 to 300,000 years ago.”
jqjacobs.net/anthro/paleo/antecessor.html
As for archaic Homo sapiens, how do we know that mEve was not herself an archaic Homo sapien? Where do we draw the distinction between the two? Don’t many modern humans have morphological features that we would attribute to being ‘archaic’?
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davidford
Guest
Rana, Fazale with Hugh Ross. 2005. Who was Adam?: A Creation Model
Approach to the Origin of Man
reasons.org/shop/customer/product.php?productid=690&cat=0&page=1
Approach to the Origin of Man
reasons.org/shop/customer/product.php?productid=690&cat=0&page=1
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Pensees
Guest
While the fact that all modern humans are traced by their mitochondrial DNA to one woman is predicted by monogenism, it was not predicted by evolution. Therefore, how could this not be considered an evidence for monogenism?
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hecd2
Guest
You are mistaken about the concept of mitochondrial Eve. She is more properly called the matrilineal Most Recent Common Ancestor of all living humans. The concept of an MRCA arises necesssarily from a consideration of genetics and population biology. The existence of an MRCA in the pure female line is a certainty (as is a different MRCA in the male line - traced by Y-chromosome coalescence, a different MRCA through the X-chromosome, a different MRCA for beta-globin gene etc). The key point about the Cann et al paper is not that they discovered the existence of a matrilineal MRCA - that was known in advance. The importance of the paper was about the date of the MRCA and the fact that it coalesced back to an African origin - that is relevant the debate about human origins (Out of Africa versus Multi-regional).
Incidentally, the person who holds the honour of being the MRCA is not constant. It is a temporary honour. From generation to generation ancient branches in the phylogeny of mtDNA become extinct and when that happens the identity of the Most Recent Common Ancestor passes to another, more recent ancestor.
Alec
evolutionpages.com
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hecd2
Guest
Of course mitochondrial Eve had a mother as has every other verrtebrate in the history of the world.
Gould is discussing the implications of the Cann et al paper for the Out of Africa versus multi-regional hypotheses. The current thinking can be called Out of Africa Again and Again (the title of Templeton’s landmark paper in Nature in 2002):
Templeton, *Out of Africa again and again, *Nature 416, 45 - 51 (2002). Abstract here:
nature.com/nature/journal/v416/n6876/abs/416045a.html
This shows that in addition to the original expansion of H erectus OOA 1.7 million years BP, there were two further expansions OOA, one between 400,000 and 800,000 years BP and one between 80,000 and 150,000 years BP. These expansions resulted in interbreeding not in replacement. So the ancestry of the modern human genome is recently complex and is not as simple as saying that we trace all our ancestry back to Africa 150,000 years ago.
She might have been an archaic H sapiens. She is right on the extreme border of the time of the appearance of H sapiens - the oldest known H sapiens are the Omo fossils which have recently been dated to 180,000 years BP.
Some specimens of archaic H sapiens hardly differ from modern people; some have moderate differences in things such as a supraorbital ridge, supraorbital sulcus, prognathous lower face, thick cranial vault, etc.
Alec
evolutionpages.com
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