600 million years ago, a single biological mistake changed everything

January 12, 2016 6:51 pm

 

Scientists resurrected the genomes of long-dead creatures based on their modern descendants’ DNA. Photo / Getty Images

If life is effectively an endless series of photocopies, as DNA is
transcribed and passed on from one being to the next, then evolution is
the high-stakes game of waiting for the copier to get it wrong.
Too wrong, and you’ll live burdened by a maladaptive mutation or genetic disorder. Worse, you might never live at all.
But
if the flaw is wrong in exactly the right way, the incredible can
happen: disease resistance, sharper eyesight, swifter feet, big brains,
better beaks for Darwin’s finches.
In a paper published in the
open access journal eLife this week, researchers say they have
pinpointed what may well be one of evolution’s greatest copy mess-ups
yet: the mutation that allowed our ancient protozoa predecessors to
evolve into complex, multicellular organisms.

Thanks to this mutation – which was not solely responsible for
the leap out of single-cellular life, but without which you, your dog
and every creature large enough to be seen without a microscope might
not be around – cells were able to communicate with one another and work
together.
And, incredibly in the world of evolutionary biology,
all it took was one tiny tweak. One gene, and complex life as we know it
was born.
“It was a shock,” co-author Ken Prehoda, a biochemist at the University of Oregon, told The Washington Post.
“If
you asked anyone on our team if they thought one mutation was going to
be responsible for this, they would have said it doesn’t seem possible.”
The
discovery was made thanks to choanoflagellates – tiny balloon-shaped
creatures that are our closest living unicellular cousins – and a cool
bit of evolutionary time travel known as ancestral protein
reconstruction, which allows scientists to resurrect the genomes of
long-dead creatures based on their modern descendants’ DNA.
In
this case, the reconstruction took Prehoda and his colleagues back some
600 million years, when ancient beings no bigger than a single cell swam
through vast shallow seas covering what are now continents. There’s
pretty much no fossil record from this period – what kind of fossil
could be left by something smaller than a pinhead? – so insights into
life at that time rely on researchers’ imaginations and intense scrutiny
of modern DNA.
For this, the choanoflagellates were perfect.
They’re single-celled organisms, but they occasionally work together in
groups, swimming into a cluster with their flagella (tails) pointing
outward like the rays of a sun. At the most basic level, this
coordination helps the choanoflagellates eat certain kinds of food. But
it’s also an example of individual cells coming together to work as one
unit, kind of like – hey! – a multicellular organism.
Prehoda and
his colleagues began to look into what genes could be responsible for
allowing the choanoflagellates to work together.
“We were
expecting many genes to be involved, working together in certain ways,
because [the jump to multicellularity] seems like a really difficult
thing to do,” he said.
But it turned out only one was needed: A
single mutation that re-purposed a certain type of protein. Instead of
working as enzymes (proteins that facilitate reactions inside the cell)
the proteins were now what’s known as an interaction domain.
They
could communicate with and bind to other proteins, a useful skill for
cells that have decided to trade the rugged individualist life for the
collaboration of a group. In the wild world of pre-complex life, this
development was orders of magnitude better than Twitter for getting
organisms organized. Every example of cells collaborating that has
arisen since – from the trilobites of 500 million years ago to the
dinosaurs to woolly mammoths to you – likely relied on it or some other
similar mutation.
That single protein domain is now present in
all animal genomes and their close unicellular relatives, according to a
University of Oregon release. It’s probably wiggling around in you
right now, helping your various cells keep in touch.
But the
discovery of the protein offers more than just a history lesson, the
researchers say. It may also have ramifications for modern medicine.
Cancer
and many other diseases, Prehoda explained, can in some ways be thought
of as cells that forgot that they’re part of a multicellular being and
have stopped communicating or taking directions from the body they
belong to. If that’s the case, then understanding what equips a cell
with the proteins to communicate might help suss out why they stop.
“That’s a very different paradigm for thinking about diseases like cancer,” Prehoda said.
“It
could allow us to think about new ways to develop therapies by focusing
on genes that are involved in this unicellular to multicellular
process.”

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