Ancient Humans Had More DNA Than We Have Now

A new atlas of human genetic diversity reveals what human ancestors’ DNA may have looked like before people migrated out of Africa.

Ancestral humans carried 40.7 million more DNA base pairs than people do today, researchers report online August 6 in Science. That’s enough DNA to build a small chromosome, says study coauthor Evan Eichler, an evolutionary geneticist at the University of Washington in Seattle.

Source: Ancestral humans had more DNA

The news isn’t that surprising – we’ve know that the Y chromosome is shrinking.  The media initially portrayed this as doom saying, that men were going to disappear…

How does DNA get shorter?  DNA in chromosomes can move around, which leads to a bunch of different methods that leads to deletion.

One such example is during crossing over, of which you may be aware. But as a crude refresher/description: homologous chromosomes line up during meiosis (mitosis as well, but meiosis occurs in sex cells, which is how we inherit DNA). Your paternal chromosome and maternal chromosome pair up, and since they’re homologous (but not identical), the tips of each chromosomes can cross over/switch. Now you have two new homologous chromosomes.

However, sometimes, when they line up, they don’t line up perfectly, and a larger portion will cross over with a shorter portion. Then, the new chromosomes will be of different size: one will have an addition, and the other will have a deletion.

These additions/deletions can cause mutations/disease, but they can be silent too.  Also, the ends of chromosomes (telomeres) aren’t replicated during cell division.  So after many cycles, chromosomes get shorter and shorter.