Goat wrote:What ISN”T predicted by the ‘creation model’ is the fact that the microcephalin common ancestor is from 1 million years ago, a full 800 years before the ‘mt-eve’ ancestor’…
I’m not sure what you mean by “a full 800 years”.
Also, the microcephalin common ancestor is not a fact. The authors state:
By using the interhaplogroup divergence test, we show that haplogroup D likely originated (emphasis mine) from a lineage separated from modern humans ~1.1 million years ago and introgressed into humans by ~37,000 years ago.
But this is problematic because if microcephalin introgressed into humans ~37,000 years ago, did humans not have large brains before then?
Their dating methodology also assumes humans and chimpanzees share a common ancestor.
First, the MRCA of all of the chromosomes in the sample was obtained by using chimpanzee sequence as the outgroup. The average sequence divergence separating the MRCA and each of the chromosomes was then calculated. Last, this average divergence was scaled to mutation rate as obtained from human-chimpanzee divergence in the region to produce coalescence time.
Dating was generated by using a simulation software that they wrote.
All software programs developed for the study are available upon request. The coalescent process as implemented in the ms software (31, 32) was used to simulate genealogies under the following parameters …
In regards to microcephalin, it states:
The gene microcephalin is a critical regulator of brain size. In humans, loss-of-function mutations in this gene cause a condition known as primary microcephaly, which is characterized by a severe reduction in brain volume (by 3- to 4-fold) but, remarkably, a retention of overall neuroarchitecture and a lack a overt defects outside of the brain (26). The exact biochemical function of microcephalin has yet to be elucidated, but this gene likely plays an essential role in promoting the proliferation of neural progenitor cells during neurogenesis (26). microcephalin has been shown to be the target of strong positive selection in the evolutionary lineage leading from ancestral primates to humans (27, 28). This observation, coupled with the fact that this gene is a critical regulator of brain size, suggests the possibility that the molecular evolution of microcephalin may have contributed to the phenotypic evolution of the human brain (27, 28).
A mutation of a gene that causes a small head does not necessarily mean that the mere presense of that gene would cause a larger head.
Further a large head/brain by itself does not mean that something is more intelligent.
In this study, we investigate the origin of the microcephalin D allele in modern humans. We show that the D allele is unlikely to have arisen within a panmictic population. Instead, our data are consistent with a model of population subdivision followed by introgression to account for the origin of the D allele. By this model, schematized in Fig. 4 B, the lineage leading to modern humans was split from another Homo lineage, and the two lineages remained in reproductive isolation for ~1,100,000 years. During this period of reproductive isolation, the modern human lineage was fixed for the non-D allele at the microcephalin locus, whereas the other Homo lineage was fixed for the D allele. These two alleles are differentiated by a large number of sequence differences accumulated during the prolonged isolation of the two populations. At or sometime before ~37,000 years ago, a (possibly rare) interbreeding event occurred between the two lineages, bringing a copy of the D allele into anatomically modern humans. Whereas the original D-bearing Homo population had since gone extinct, this introgressed copy of the D allele in humans had subsequently spread to exceptionally high frequency throughout much of world because of positive selection.
But even if microcephalin is a factor in the development of a larger brain because of associations with microcephaly, then it would only be one of six genes that would be necessary.
“Microcephalin (MCPH1) is one of six genes causing primary microcephaly”
A derived form of MCPH1 called haplogroup D appeared about 37,000 years ago (anytime between 14,000 and 60,000 years ago) and has spread to become the more common form throughout the world except Sub-Saharan Africa.
Haplogroup D may have originated from a lineage separated from modern humans approximately 1.1 million years ago and later introgressed into humans.
Shouldn’t MCPH1 be common among all humans living in the world, not just particular regions of the world?
Our results not only provide genetic evidence in support of the possibility of admixture between modern humans and an archaic Homo lineage but also support the notion that the biological evolution of modern humans might have benefited from the contribution of adaptive alleles from our archaic relatives.
If MCPH1 causes a reproductive advantage, how can it be explained the ancestor of it has become extinct?