Introgression from Gorilla caused the Human-Chimpanzee split (Preprint)
Authors: Nygren Johan 
Published: May 2020

BACKGROUND

ABSTRACT: The Gorilla Genome Project (Scally, 2012) showed that 30% of the gorilla genome introgressed into the ancestor of humans and chimpanzees, and that the two species diverged through lineage sorting with 15% ending up in Pan and another 15% in Homo. That introgression is the Pan-Homo split, hybridization, which led to speciation as the new hybrid lineages became reproductively isolated from one another.

The NUMT on chromosome 5 (“ps5”) (Popadin, 2017) fits perfectly with the introgression speciation model, it was formed from mtDNA that had diverged from the common ancestor of Pan-Homo for 1.8 Myr at the time of insertion into the nuclear genome, and originated in the Gorilla lineage. The ps5 pseudogene was transferred to Pan and Homo during the introgression event that led to the Pan-Homo split, 6 million years ago.

OBJECTIVE

Genome sequencing has been evolving along the law of accelerating returns (Kurzweil, 2004), the total amount of sequence data produced doubling approximately every seven months. (Stephens, 2015) With the genetic revolution, phylogenetic relationships are no longer limited to morphological characters, they can instead be read like an open book. This thesis will explore a new chapter, with roots in genetic data from the Gorilla Genome Project (Scally, 2012).

The Gorilla Genome Project was the first complete genome of Gorilla, from a female western lowland gorilla, and it revealed a closer relationship between humans and gorilla than what morphological analyses had shown: in 30% of the genome, gorilla is closer to human or chimpanzee than the latter are to each other. At the time interpreted as incomplete lineage sorting (Scally, 2012), genetic evidence of gene transfer between Gorilla, Pan and Homo around the time of the Pan-Homo split (Popadin, 2017) shows that the lineage sorting is more parsimonious as a result of introgression.

Introgression may lead to speciation, in which the new hybrid lineages become reproductively isolated from parental populations (Baack, 2007), and since Pan and Homo have diverged through lineage sorting, with 15% of the introgressed genes ending up in Pan and another 15% in Homo, it is reasonable to conclude that the introgression caused the Pan-Homo split (Fig. 1), and therefore that it occurred at the time of the Pan-Homo split, around 6 million years ago.

METHODS

Phylogenetic relationships can be read from genome comparison. That there was gene transfer between Gorilla, Pan and Homo around the time of the Pan-Homo split can be read from a NUMT on chromosome 5 (ps5), which diverged between Gorilla, Pan and Homo at the time of the split. (Popadin, 2017) The ps5 NUMT as evidence of gene flow shows that introgression is a more parsimonious explanation for the lineage sorting from Gorilla than incomplete lineage sorting (ILS), and since Pan and Homo diverged through lineage sorting, it can be read that the introgression caused the Pan-Homo split. (Fig. 1)

RESULTS

The lineage sorting of 30% of the gorilla genome that is seen in humans and chimpanzees (Scally, 2012) is a result of introgression, an event that caused the speciation of Pan and Homo (Fig. 1), and the two lineages diverged through lineage sorting with 15% of the introgressed genes ending up in Pan and another 15% in Homo

CONCLUSIONS

The indisputable evidence that an introgression event caused the speciation of Pan and Homo is made possible by the genome revolution, and it provides a map, a reference frame, that makes it possible to read the world in ways that were previously out of sight, and can provide an important reference for continued research into hominin evolution. The fossil record shows that there were multiple lineages of hominin coexisting throughout the Pliocene and Pleistocene (Haile-Selassie, 2016), and the introgression speciation model can provide clues to how those lineages relate to one another. What remains to be understood is what environmental and ecological factors triggered the hybridization.