It was the evolutionary "big bang" that started them all. Scientists have found the earliest evidence of sexual reproduction in Arctic rocks 1.2 billion years old.
A detailed analysis of fossilised red
algae has pushed back the time when the battle of the sexes commenced and complex life forms first emerged.
The research by Nicholas Butterfield, a palaeontologist at the University of Cambridge, supports the theory that the
two events were intimately linked.
His findings were unveiled this week
at the Palaeontological Association meeting in Manchester and will be detailed in the journal Paleobiology.
"This is the first indication of sex
on earth and the oldest instance of complex, multicellular life," said Dr Butterfield.
The first life forms to appear on earth, at least 3.5 billion years ago, were bacteria that reproduced by cloning themselves just as they do today.
However, another branch in the tree
of life, the eukaryotes, which include all animals, plants,
fungi and many micro-organisms, went on to develop sexual reproduction in which two individuals of different sexes create the next generation with a mixture of genes from both parents.
This remarkable innovation was the most significant development in the story of life after its genesis, opening
the way to the development of the
and variety of living things on earth today.
The fossils Dr Butterfield has been studying represent the closest science has been able to get to this momentous episode.
In 1.2 billion-
year-old rock nodules from Arctic Canada, he has identified
large populations of a
fossil seaweed essentially indistinguishable from a modern red alga called Bangia. Named Bangiomorpha pubescens, these are the oldest fossil eukaryotes that can be confidently assigned to a known taxonomic group.
But their significance is far more fundamental than this.
By studying many individuals at different stages of maturity, Dr Butterfield has been able to track the way the
alga developed throughout its life.
Evidence of sex was betrayed by the size and number of spores an individual alga produced.
By comparing the fossils with living relatives, Dr Butterfield recognised
two distinct types in the ancient populations: those that produced a few large spores, thought to be asexual, and those that produce many small spores, identified as sexual.
Whether the sexual plants are male or female has yet to be resolved, but the
case for sex is unambiguous.
"They are spectacular fossils,
so beautifully preserved you can actually see their life cycle within the population and you can see the cell division patterns by which you can distinguish sexes in the modern form,"
The broader significance of these fossils is that they occur at a time of major evolutionary diversification, dubbed the "eukaryotic big bang", when most of the modern kingdoms of life appear to have originated.
Dr Butterfield said he believed that this was linked to the appearance of complex multi-
cellular life, which
in turn was dependent upon the evol-ution of sex.