Gorilla-human split - implications for the evolution of deep trophoblast invasion

Lowland gorilla (Gorilla gorilla) female and young
at the Bronx Zoo - Wikipedia Commons (CC-BY-SA 3.0)

The fossil Chororapithecus abyssinicus is the earliest known species of gorilla. A recent paper (here) dates this fossil and calculates that the split between the gorilla and human lineages occurred more than 8 million years ago (Mya). This agrees well with an estimate based on genomic data (here) putting the gorilla-human split at 7.6 to 9.7 Mya.

Placenta of Gorilla gorilla showing invasion of decidua (at left) by
trophoblasts (black stain) following the interstitial route;
villi and intervillous space  are at right

We have shown that trophoblast invasion in gorilla and chimpanzee resembles that in humans. For example there is invasion by the so called interstitial route which does not occur in monkeys or in lesser apes such as gibbons (reviewed here and previous post).

Fetus of Gorilla gorilla photographed by Louis Bolk
Courtesy of Dr. Laurens de Rooy, Museum Vrolik, Amsterdam

Gravid uteri of great apes are not easy to come by; one of the gorilla placentas we saw was collected by Louis Bolk in 1923. He described the fetus (here) and sent the placenta to J. P. Hill in London. 

We have not been able to find a gravid uterus of an orangutan. The split between orangutan and other great apes was as deep as 15-19 Mya (here). It would be interesting to know whether human-like trophoblast invasion evolved before or after that.