Re: When did sex first appear in our evolutionary history?

Dear Robert,

Thanks for this very interesting question.  It's a question that is a very 
hot topic in evolutionary biology and the short answer is "we don't 
know"!  Let me quote you a passage from the excellent book "Life: An 
Unauthorised Biography", by Richard Fortey (highly recommended):

"Exactly how sexual differentiation originated is still hotly debated...it 
must be stated that it was evidently an ancient Precambrian innovation 
because so many plants and animals-even fungi-show evidence of sexual 
reproduction.  The sexual imperative runs deep in time".

The Precambrian is the period of Earth's history from 4500 million years 
ago until about 550 million years ago.  During this time the first life 
forms appeared, primitive one-celled organisms similar to some of today's 
bacteria.  This was pretty much all that there was for hundreds of 
millions of years.  Gradually more complex cells evolved into what we call 
eukaryotes (cells with a nucleus and internal structures), then the first 
simple multicellular organisms, then simple soft-bodied animals and plants 
that left no fossil record.  But we know they must have existed because 
after 550 million years ago we witness the "Cambrian explosion"-a sudden 
increase in numerous fossil species of all kinds.  These fossils show 
sexes and so the origin of sexes must be very ancient.

Before sexual reproduction, the first single-celled organisms must have 
reproduced by binary fission as bacteria and some yeasts do today-that is 
copying their genetic material and then dividing into 2 daughter cells.  
But of course even bacteria have a primitive form of sex called 
conjugation, whereby 2 cells can join and exchange genetic material 
through tubes called pili.  If we define sex as the transfer of genes 
between organisms of one species, then this must have been the advent of 
sexual reproduction.  However at some point, the simple organisms must 
have differentiated into males and females.  Presumably this occurred when 
cells started to have several chromosomes-that is, several packages of DNA 
rather than the one (or occasionally more) circular chromosome found in 
bacteria.  This would allow chromosomes to become specialised into those 
concerned with "female functions" (producing gametes, or eggs) and those 
concerned with male functions (producing sperm).  This arrangement must 
then have had sufficient selective advantage to be maintained and led to 
the development of separate sexes in all subsequent higher species.

A great deal of the debate in this area centres around "why have sex?"-in 
other words, what is the selective advantage of having sexes?  There are 
many mathematical models explaining this and these are discussed very well 
in the books of Richard Dawkins and Stephen Jay Gould.  But the trickier 
question of how and when sexes first appeared is ignored, because it's 
very difficult!  It may be something that we never know.

Neil Saunders