https://www.thetech.org/ask-a-geneticist/48-46

Alright, I tried to bring this up in the past, I know
I mentioned it, but here we go again...

So I don't know anybody who thinks that a baby
was born 800k or 2 million years ago and,
suddenly, humans stopped having 48 Chromosomes
and started having 46.

Nope.

What likely happened is that /Some/ ancestors
started having 47.

AND THEN, either they bumped into another group
that also commonly had 47, resulting in one group
with 46, or it was always just one group and then
some kind of Found Effect got them all playing
"Monster Mash" and, before you know it, everyone
was born with 46...

I'm on record favoring the multiple events scenario.

Why? Because it doubles the size of our starting
population, giving them the best chance to take off,
take over. And...

Because the fusion wouldn't be any kind of advantage,
as far as I can see. It likely would have been a
disadvantage. It wouldn't have stopped breeding but
it wouldn't have helped, either. Might've slowed things
down. But, just for the sake of argument, let's look at
the single event scenario.

To me this plays to r/K selection. Anything that hampers
reproduction, even just a little, increases the significance
of every offspring. Also...

If you had a gorilla style reproductive strategy, one male
a a bunch of females, a mutation does get the opportunity
to take root... take over.

Right?

If such a mutation crops up in a single female out of a
group, there's an excellent chance it will be drowned out
by the breeding of all the females who don't have it. And
it's the same deal in any male within a population were
males all have a go at the breeding thing.

ESPECIALLY IF THE MUTATION PRESENTS A SMALL
DISADVANTAGE IN BREEDING!

So if we're talking a single event -- the chromosome
fusion popped up in one group, and that group evolved
into modern humans -- to me it suggests that this was
not a sexually selected group, that a lot of investment
went into each offspring and we did not have a "Quantity
has a quality all of it's own" reproductive strategy.

Or, this had to become the case VERY quickly, once the
fusion happened...

::Discuss::




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