How did sexual reproduction originate and evolve?

I was recently asked how sexual reproduction originated and evolved to the point where we now have sperm, eggs, etc. The point of the question was to demonstrate that the steps required to get to our current reproductive systems are too complex to have formed via evolution. It sounded like a specific example of ‘irreducible complexity’. This question was given as evidence that micro-evolution may occur, but macro-evolution doesn’t happen. I have found some discussions online about the advantage sexual reproduction has over asexual reproduction, but I didn’t find anything specific to the origination and evolution of sexual reproduction. Are there studies, theories, etc for the evolution of sexual reproduction?

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Sex is thought to have originated once, in the lineage leading to the common eukaryote ancestor, so we’re talking about something that happened a long, long time ago. So, many of the issues surrounding this topic will be similar to origin of life issues, and used in similar ways by anti-evolution types. For comments on this general approach, you could read the back-and-forth I had with Stephen Meyer some time ago in the ASA journal, specifically my reply here:

On the current science on the origins of sex, here’s a recent review:




i dont think it possible. think about 2 self replicating robots that have a dna and a self rplicating system. do you think that one robot can evolve a (complex) system that need another complex system (with another robot) to make it work? i think that its impossible. because we cant for example design a gps system that base on one part. so a system that is more complex its mission impossible.

okay, 2 self replicating robots is an improvement over 2 self replicating Jeeps.

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@DennisVenema I have a question, kind of related: Do scientists have any idea of when humans stopped having estrus/heat cycles?


Well, we still do in a way - human females of reproductive age cycle about once per month. What humans have is an estrus cycle that is not a once-per-year event, and we have what is called ovulatory crypsis - females do not advertise that they are fertile to males with overt biological signs. I’m not 100% sure but it seems to me I recall that these features arose on the hominin lineage (i.e. after our lineage diverged from chimpanzees).


Sorry don’t know where to put this one:

Thank you. I guess that’s what I meant – the overt biological signs including odor.

This is a very interesting area of investigation. There are those who think this shift from an annual “estrus” to a “veiled cycle” when adults started spending extending periods of time in a marine environment (the so-called bottle neck at the African river). Males and females lost most of their body hair with the exceptions of the pubic patches (presumably to broadcast pheromones). But the relevance of even this residual hair would have become eclipsed by the real breakthrough of the veiled estrus cycle.

If a woman might mate with you at any time during the year . . . the incentive to bond with her goes WAYYY up! [Until you get married? :smiley: ]

Female survival and reproduction improved as males became more attentive to females (and more consistently protective) with sexual rewards expanding to year-round, instead of just once a year.

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