Hacker News Reader

Comment by arnidg | Hacker News Reader

> Are we a result of similar microorganisms?

I'm no biologist, but from what I've read: Probably not. A really important fact about multicellular life is that each cell is genetically identical to any other cell in the organism. Otherwise, different cells will start competing with one another, until the relationship falls apart. In nature, symbiosis only works because each cell has an independent means of reproduction, and that always puts a limit to how close the cooperation can be (nothing close to forming multicellular life).

View on HN

If you were to pull a random cell from your body, there's a 90% chance it doesn't contain human DNA: https://www.scientificamerican.com/article/strange-but-true-...

A bit clickbait-y. Do you consider your digestive cavity a part of your body or is your body all the stuff around it?

Maybe that statement should come with an explicit topological constraint on the meaning of “inside” the body. Viewed as a donut, gut contents would be considered “outside.”

> A really important fact about multicellular life is that each cell is genetically identical to any other cell in the organism.

Not necessarily. https://en.wikipedia.org/wiki/Chimera_%28genetics%29#Humans

Our cells DNA also drift over time due to localized mutations. What matters is that they should keep obeying all the chemical signalling.

> Otherwise, different cells will start competing with one another, until the relationship falls apart.

When that happens in complex multicellular organisms, it's usually called "cancer".

> In nature, symbiosis only works because each cell has an independent means of reproduction, and that always puts a limit to how close the cooperation can be (nothing close to forming multicellular life)

Mitochondria is its own organism, has its own DNA that is separate and distinct from nuclear DNA. It has been very successful through very close cooperation.[0]

[0] https://en.wikipedia.org/wiki/Symbiogenesis

True, though largely limited to DNA necessary for its functions. I don’t have a citation on me (mobile) but there’s evidence that more “generic” mitochondrial DNA was integrated into the nuclear DNA, and that this is also the case for other endosymbionts.

> and that this is also the case for other endosymbionts.

such as chloroplasts.

I have a hypothesis for why this happens. Sexual reproduction has a very neat property: Recombination. Two individuals that each have one harmful mutation can through recombination have offspring without either of them. This allows removal of harmful mutations from the gene pool without terminating someones entire lineage - important when every generation comes with a decade of mutations, unlike microorganisms that are more on the scale of hours or days. However mitochondrial DNA cannot recombine, so it cannot benefit from this mechanism. Therefore it makes sense to move as much DNA as possible from the mitochondria to the nucleus. The same goes for the Y-chromosome, and could explain why it has been losing genes over time at a truly astounding pace.

> In the last 190 million years, the number of genes on the Y has plummeted from more than 1,000 to roughly 50, a loss of more than 95 percent.

https://www.quantamagazine.org/the-incredible-shrinking-sex-...

> > In the last 190 million years, the number of genes on the Y has plummeted from more than 1,000 to roughly 50, a loss of more than 95 percent.

How much of this has happened in the recent human history? Are we becoming less “manly”?

Not really. The y-chromosome started out as a tiny variation of the x-chromosome. So all those genes were duplicates of genes on the x-chromosome.

They are not. Trees are notorious to have variations across their branches.

Reply on news.ycombinator.com