Mini-lecture: The origins of complex life (UCL)

Uploaded by UCLTV on 04.12.2009

My name's Dr Nick Lane. I'm in the Department of Genetics, Evolution & Environment
at University College London.
And I'm interested in the
ways in which complicated life, complex life, arose on Earth.
In fact, there's a big hole right at the centre of biology because
complex life arose on our planet only once
in 4 billion years of history. We know that because
all plants, all animals
algae, fungi and so on, everything you can see, everything which is important to us on this planet
shares a basic cell structure. And that cell structure- there are so many points in common that
it's very plain that it all had a single ancestor. And that single ancestor
arose once
from the bacteria. And it's very different
to bacteria today, to prokaryotes, which are very simple, very small cells
and they arose perhaps 3.5, maybe 4 billion years ago.
And they've hardly changed in that whole time. We can find fossils four billion years old,
3.5 billion years old,
of bacteria that look the same as bacteria today, so I'm interested in what it was,
what event
led to this transformation
of possibilities
from the very simple
morphologically simple bacterial cell
to the enormously complex
eukaryote cell, which gave rise to all these different explosion of organisms.
I think that the reason for this is that there was a very unusual event between
two bacteria, perhaps 2 billion years ago, something like that, where one got inside another one.
It was a chimeric event to give rise to some kind of fusion of two cells. Now this kind
of thing happens very rarely
between bacteria. But we know of the odd example.
And I think that this case rise to this huge
and the reason it relates to
the way in which bacteria breathe--
I think they breathe across their skin, across their outer membrane.
And that limits how large they can get because
the larger they get, the less surface area they have in relation to their volume.
And the only way out for them is to internalise
their breathing inside.
And the problem with that
is that they require genes to control it and the bigger a bacteria is,
the more copies of its genome it has. And that's a tremendously
problem for it. They simply can't get around it.
Having one cell inside another cell suddenly gives rise to this state where
you're able to expand up in size
while being stable as a cell
and while having lots and lots of energy. So that's the difference really as I see it between
the bacteria which are limited by energy
and the eukaryotes, which are able to expand and swell and accumulate DNA. And they're
not limited in energy.
And that gives
a second problem, which is that all eukaryotes have to have all these multiple copies
of the small genome, a tiny genome, the mitochondrial genes
to support their state
to exist at all. They have to have an interaction
between these genomes. So to talk about a single genome in a cell is not right. There are 2 genomes in
every cell.
And the way in which they interact with each other
is critical for the way that all complex life
and underpins,
for example,
why we get old, why we age, why we have
sex, why there are two sexes instead of everybody could be one sex,
problems with fertility,
why cells commit suicide- all these kinds of questions can be explained quite simply
in terms of the requirement for having two genomes
in every single cell.
So I think this is why
complex life arose only once
on Earth
as a result of a unique
fusion between two bacteria,
two billion years ago,
and this explosion then after that giving rise to all complex life on Earth, along with their very
peculiar properties,
can be explained quite simply in terms of the interaction
of these two genomes.