Introgression from the Ancients

As stated in my previous blog, it appears that we were not the first “human-like” species to leave our mother-country of Africa. The Neanderthal and Denisovan people made this migration before our ancestors did. However, the Neanderthals and Denisovans may have played a role in our own ancestry. Unless your heritage comes solely from Africa, it is likely that you have a bit of Neanderthal DNA in you. This is partially because our ancestors mated with the Neanderthals in the Near and Middle East as they were leaving Africa. This caused major introgression events, meaning that the Neanderthals introduced non-human genetic material into the human population migrating out of Africa.

(Click Image to Enlarge – Image taken from Wikipedia article “History of human migration”
The numbers on this map represent how many years ago the human migrations occurred.
Homo erectus* – A hominin species whose DNA has not yet been sequenced)

Remnants of these introgression events (as well as others that took place at different points in time when the two groups came in contact with one another) can be spotted in our genomes. In fact, over half of the Neanderthal genome can be recreated using the DNA of today’s modern humans. This high percentage is due to an additive effect from the fact that each person with a heritage from outside of Africa gets approximately 1-3% of their DNA from Neanderthals. Keep in mind, your 1-3% might not be the same as my 1-3%. It simply depends on what part of one’s genome the Neanderthal bits survived in until today.

A similar story can be told with the Denisovan people. Introgression events granting Denisovan DNA to our ancestors occurred as our people spread southeast throughout Asia. Nowadays, it is generally the Melanesians, Papuans, and Australians that have the largest segments of DNA that come from the Denisovan genome. Approximately 3-6% of their ancestry comes from the Denisovans.

You may now be wondering why our genomes still contain segments of Neanderthal/Denisovan DNA. Some scientists will attribute the majority of this to genetic drift, meaning that it is entirely due to chance (**see previous blog post – “Survival of the Fittest? Not Always…”***). However, certain fragments may still be in our genome due to a process known as adaptive introgression. According to Racimo et al., “As modern and ancient DNA sequence data from diverse human populations accumulate, evidence is increasing in support of the existence of beneficial variants acquired from archaic humans that may have accelerated adaptation and improved survival in new environments.” This states that some of these segments from Neanderthals/Denisovans may be in our genomes because they were beneficial enough to help our ancestors thrive and reproduce successfully.

For my next blog post, I will be changing gears to talk about a scientific procedure known as Polymerase Chain Reaction, or PCR for short. The discovery of this simple technique allowed for DNA studies to blossom and allow for an array of studies. These range from the study of ancient DNA, such as that of the Neanderthals and Denisovans, all the way to the forensic analyses used so frequently in America’s judicial system. Until next time, carry on with curiosity.

 

Works Cited

Racimo, F., Sankararaman, S., Nielsen, R., & Huerta-Sánchez, E. (2015). Evidence for archaic adaptive introgression in humans. Nature Reviews Genetics16(6), 359-371.

History of human migration. (2016, June 24). In Wikipedia, The Free Encyclopedia. Retrieved 20:47, July 13, 2016, from https://en.wikipedia.org/w/index.php?title=History_of_human_migration&oldid=726769187

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s