Three hundred million years ago there was a chromosome.
It had no name, let’s call it X, though just an autosome.
It had a partner just the same but don’t say they were gay.
To tell the truth, they didn’t care, for both went either way.

Then some genes flipped; some genes flopped; from one X, parts were lopped.
Birds from crawlers flew away; monotremes got underway.
It’s still that way in platypus, not male nor female-like.
The Duckbill X and partner look pretty much alike.

A hundred fifty million years went by and nothing changed.
But then the X’s partnership became once more deranged.
Another sequence flipped and flopped and lost more DNA
And that’s the situation in marsupials today.

When some genes flipped; some genes flopped; from one X, parts were lopped,
Parents of the kangaroo started hopping two by two.
The wallaby, koala and opossum pioneers
Chopped Xs down to Y size and modified their rears.

Another fifty million years went by, then once again
Another mass inversion struck the disappearing twin.
That’s what we see in mammals all around the world today,
Unless we look at primates, where there’s more change on the way.

When some genes flipped; some genes flopped; from the Y, parts were lopped,
Mammals dominate our tale, tiny shrew to mighty whale,
With sexes split like ours are: XYs make the bulls and rams
And those that have two Xs become the does and dams.

Then forty million years ago, a fourth inversion clipped
The Ys to tiny fragments of the X, whence they were snipped.
Just nineteen ancient genes remain from X’s former span.
But we’ve not lost the X in vain, its dreck has made a man.

So, some genes flipped; some genes flopped; from the Y, parts were lopped.
Will there come a future when there’s no Y and hence, no men?
I know the Bible story says that Adam’s rib made Eve.
But now you’ve heard the data, so which do you believe?

I know the Bible tells us Eve was made from Adam’s rib
But Adam’s evolution looks more like an ad lib.