How did male and female first evolve?

Kat Arney put this question to Ginny Smith...

Ginny - Yes, well. If we're going to talk about the origins of male and female, we do actually have to start by talking about the origins of sex because, of course, when at first it was all single celled organisms, they mainly reproduced through asexual reproduction. Bacteria, yeast, things like that still do it.

Kat - They split in two, don't they? One cell becomes two, becomes four...

Ginny - It's very, very quick. It's very, very easy. You don't have to bother finding someone else to do it with...

Kat - And buying them dinner and all that kind of stuff...

Ginny - So why did we stop doing it? Well the problem is, bad mutations can build up because you're just producing a clone of yourself. And it also means, if something new comes along - a new pathogen or a new environmental influence, it's going to wipe out the whole line. So it turned out that mixing your genes with another animal's genes is a much better way of producing offspring that are likely to survive. So sex was one of the best ways of doing this, and the first sexual encounters were likely to have been between two organisms that were effectively the same, not male or female, both somewhere inbetween. They just started swapping some information.

Kat - Because bacteria do this. They swap little things called plasmids. Tiny little bits of DNA they just sort of go - do you want some of that, here you go. And this is usually how antibiotic resistance spreads, doesn't it? It's carried on plasmids.

Ginny - Exactly, exactly. So that's kind of the first precursor to real sex. But, over time, many species then evolved to having two mating types - call them type A and type B. Now, we aren't quite sure why this happens, but it might be something to do with keeping our mitochondria in check because only one of the two parents now passes down mitochondria. We get all our mitochondria from our mother, so it may be that A and B evolved because one of them passed down the mitochondria and the other one didn't. Now once this had happened, it was an advantage for each of the mating types to specialise. So one type started making more smaller gametes and that meant that they could have more offspring because they could mate more easily with more of them, and the other one started putting all it's resources into ensuring the health of the offspring and the survival of the offspring they produced.

Kat - All it's basket in one egg!

Ginny - Exactly, yes. So that then over lots of time as they became more and more specialised...

Kat - Give it a billion years, and here we are...

Ginny - Became male and female and that, of course, leads to the choosy female - ardent male system that we see in most animals now. And, interestingly, there's some evidence that there's a real advantage to species that do that because it ensures that only the fittest males get to pass on their DNA and it improves the fitness of the next generation as a whole.

Kat - And certainly, as a woman, it means I get taken out for nice dinners!