ORIGIN OF SEX DISCOVERED
A profound new
discovery announced in Nature today by palaeontologist, Flinders
University Professor John Long, reveals how the intimate act of sexual
intercourse first evolved in our deep distant ancestors
In one of the biggest
discoveries in the evolutionary history of sexual reproduction, Professor Long
has found that internal fertilisation and copulation appeared in ancient
armoured fishes, called placoderms, about 385 million years ago in what is now
Scotland.
Placoderms, the most
primitive jawed vertebrates, are the earliest vertebrate ancestors of humans.
Published in Nature,
the discovery shows that male fossils of the Microbrachius dicki,
which belong to the antiarch group of placoderms, developed bony L-shaped
genital limbs called claspers to transfer sperm to females; and females
developed small paired bones to lock the male organs in place for mating.
Measuring about 8cm
long, Microbrachius lived in ancient lake habitats in
Scotland, as well as parts of Estonia and China.
As the paper's lead
author, Professor Long, who is the Strategic Professor in Palaeontology at
Flinders University in South Australia, discovered the ancient fishes mating
abilities when he stumbled across a single fossil bone in the collections of
the University of Technology in Tallinn, Estonia, last year.
The fossils, he said,
symbolise the most primitive known vertebrate sexual organ ever found,
demonstrating the first use of internal fertilisation and copulation as a
reproductive strategy known in the fossil record.
"Microbrachius means
little arms but scientists have been baffled for centuries by what these bony
paired arms were actually there for. We've solved this great mystery because
they were there for mating, so that the male could position his claspers into
the female genital area," Professor Long said.
"It was
previously thought that reproduction spawned externally in water, and much
later down the track in the history of vertebrate evolution," he said.
"Our earlier
discoveries published in Nature in 2008 and 2009 of live birth
and copulation in placoderms concerned more advanced placoderm groups. Our new
discovery now pushes the origin of copulation back even further down the
evolutionary ladder, to the most basal of all jawed animals.
"Basically it's
the first branch off the evolutionary tree where these reproductive strategies
started."
In one of the more
bizarre findings of his research, Professor Long said the fishes probably
copulated from a sideways position with their bony jointed arms locked
together.
"This enabled the
males to manoeuvre their genital organs into the right position for mating.
"With their arms
interlocked, these fish looked more like they are square dancing the do-se-do
rather than mating."
Flinders Postdoctoral
Research Fellow Dr Brian Choo, a co-author on the paper, said the discovery
signifies the first time in evolutionary history that males and females showed
distinct differences in their physical appearance.
"Until this point
in evolution, the skeletons of jawed vertebrates couldn't be distinguished
because males and females had the same skeletal structures," Dr Choo said.
"This is the
first time in vertebrate evolution that males and females developed separate
reproductive structures, with males developing claspers, and females developing
fixed plates to lock the claspers in for mating," he said.
The discovery
highlights the importance of placoderms in the evolution of vertebrate animals,
including humans, Professor Long said.
"Placoderms were
once thought to be a dead-end group with no live relatives but recent studies
show that our own evolution is deeply rooted in placoderms, and that many of
the features we have, such as jaws, teeth and paired limbs, first originated
with this group of fishes.
"Now, we reveal
they gave us the intimate act of sexual intercourse as well."
Dr Matt Friedman, a
palaeobiologist from the University of Oxford, UK, described the discovery as
"nothing short of remarkable."
"Claspers in
these fishes demand one of two alternative, but equally provocative, scenarios:
either an unprecedented loss of internal fertilisation in vertebrates, or the
coherence of the armoured placoderms as a single branch in the tree of
life," Dr Friedman, who was not involved in the study, said.
"Both conclusions
fly in the face of received wisdom, and suggest that there is still much to
discover about this critical episode in our own extended evolutionary
history."
The research involved
a team of collaborators from Australia, Estonia, the UK, Sweden and China, who
scrutinised a vast number of fossil specimens held in museum collections across
the world.
Fossil specimens of
male and female Microbrachius fossils will be placed on public display in the
foyer of the South Australian Museum from today (October 20).
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