They dont have any mystical, magical, purposeful functions.
They arent ABSOLUTELY NECESSARY for anything, and technically, they cause trouble when they happen to plop themselves upstream of a gene and are thus negatively selected against, but I digress…
LINEs are kinda relatives of retroviruses that got trapped in our genome. Our genome doesnt really have an efficient way of cutting them out, though our genomes do try to keep LINEs quiet via epigenetics, and usually LINEs are totally non-functional due to mutations. And LINEs dont disappear on their own– cutting themselves out of a genome is not part of their life-cycle. They copy themselves, and plop down in a new spot (copy/paste, not cut/paste). Selfish DNA.
So we have accumulated hundreds of thousands of these guys in our genomes.
I just read a neato paper where they used high throughput sequencing to find/compare human-specific LINEs in 25 people.
They conclude that:
- LINEs are super useful for determining/tracing human ancestry. As Im sure you all have heard me mention before, new bits of junk that arent ‘supposed’ to be there make excellent connect-the-dots pictures for ancestry (this isnt new, but its still neat).
- Any two people had ~285 differences in human-specific LINEs (148 minimum, 422 maximum). I have some you dont have, you have some I dont have.
- They did not check to see whether these LINEs were homozygous (in both copies of your genome). There is probably even more diversity here– I have some LINEs you dont have, but some of them are just on one of my two copies of DNA in each cell. Since we only pass on one copy of our DNA to our offspring, this has an effect on how fast/whether new LINEs are fixed in a population or not. Genetic drift, w00t!
- The older the LINE, the more likely we all have it in common (we are all related!).
- Brand spanking new LINEs pop up in ~1/140 births. So, of the ~6 billion people alive today, there are ~30 million new LINEs between us all. If all of these junk LINEs are precious and specially created by Teh Designer, all of us without the new sacred LINEs should be dead. We aint. So…
Its just junk, but its still neato!