2023-11-02 09:00:00
It is known that other hominids (orangutans, chimpanzees and gorillas) have 48 chromosomes, but recent humans have 46. And the two we are missing? Well, it turns out that these merged and ended up becoming one.
These chromosomes are the second and third hominid pair, called 2A and 2B, which fused to give way to human chromosome 2. This is proven, since human chromosome 2 has vestiges of a second centromere (the region through which ” to chromosomes during cell division), but is inactive.
In addition, there are telomeric regions in the middle of chromosome 2 and these regions are the “poles” of the chromosomes. Both pieces of evidence make it clear that chromosome 2 is the product of the fusion of chromosomes 2A and 2B of non-hominids. humans.
When did this merger occur?
But when did this merger occur? Recent research suggests that this took place around 900,000 years before the Present (BP). This was a milestone in research, since it was assumed that because mutations in telomeres and centromeres are very sporadic, these regions did not serve to “molecularly date” when the chromosomes fused.
However, the research team that found the date did not look for mutations per se, but rather with a weight model for replacements of the nitrogenous bases of DNA (where a replacement of purines is more energetically expensive than one of pyrimidines). He found this date.
So far so good, until this year a paleo-demographic study was published using DNA to trace the history of human populations and the surprise is that around 900,000 years BP the human population was drastically reduced and went from having 98,130 reproductive individuals to only 1,280 reproductive individuals, which remained in that super-reduced number for about 117,000 years… After millennia, the human population emerged from that bottleneck to establish itself with 27,160 reproductive individuals.
It is known that the species that entered this bottleneck was Homo erectus. They fused their chromosomes 2A and 2B to now have only chromosome 2. This new population does not have a specific name, but it could be the debated Homo heidelbergensis, which gave rise to the Neanderthal-Denisovan and modern human (Homo sapiens) lineages. parallel. However, research is still lacking in this area.
So it was a milestone to fuse chromosomes, but this is consistent with the most severe bottleneck on record that ALMOST extinct our ancestors.
Main sources:
Poszewiecka, B., Gogolewski, K., Stankiewicz, P., & Gambin, A. (2022). Revised time estimation of the ancestral human chromosome 2 fusion. BMC genomics, 23(6), 1-16.
Hu, W., Hao, Z., Du, P., Di Vincenzo, F., Manzi, G., Cui, J., … & Li, H. (2023). Genomic inference of a severe human bottleneck during the Early to Middle Pleistocene transition. Science, 381(6661), 979-984.
GRADES:
1. The reason why chromosome fusion appears to be linked to this bottleneck is unknown.
2. We know that correlation is not the same as causation, but apart from the fact that it is not us, but the authors of the paper who affirm this, it is always done from the informed hypothesis, not from the dogmatic assertion.
Maybe it’s a HUGE coincidence, but usually, when there are events this important, it’s not.
Source: Palaeos
#Chromosome #fusion #ancestors #extinct