>> So the fact is is that we don't really
know how life came to be on planet Earth. It's always interesting and fun to
talk about the cutting edge of science. And that is the limit of our understanding. Just like the early microbiologists were at the
limit of their understanding when they began to realize that diseases
were caused by microorganism. So we -- there is -- even know
there are still plenty of limits, scientifically, to our understanding. But it's been a lot of fun,
and will continue to be fun, to see the scientific community
explore these ideas. Now there is evidence of early life. So we said earlier that around 3.5 billion
years ago, life certainly was on planet Earth. And the reason we know this is because,
amazingly, there are fossils of bacteria. And I'm going to show you some examples
of stromatolites, and sulfur bacteria, and cyanobacteria that support the
presence of life 3.5 billion years ago. The other evidence that we can look at for
early life is just the geological record. Just like humans are changing the planet
now, and causing it to warm very rapidly, early bacteria changed the chemical
composition of early Earth as well. So we'll talk about both of those ideas. These are some fossils that have sulfur bacteria that were discovered just a couple
of years ago, 2014, I think. So five years ago. And these actually pre-date the presence of
oxygen on Earth, which is really incredible. They're fairly large bacteria, and I don't
actually know what we're looking at here. But this is a cool 3D --
this is from NASA's website where they're basically showing
you the presence of these bacteria. And based on the microns, I believe each of these is a little bacteria
embedded in a sample of rock. Each of these little specks
that they're showing you. Now we also have known for a long time
about a structure called a stromatolite. We have stromatolites on Earth,
they're about 3.5 billion years old. And what these are is layers
of bacteria that form -- and these days we find them
in the form of cyanobacteria. And these bacteria don't exist -- I mean,
they are single cells but they don't exist as single cells, they exist
as structures called biofilms, where they're all sort of attached together. And they form this layers, these
biofilms are these layers of bacteria. And in between the layers, sediment is trapped. And so you get these minerals trapped and
forming rock over many millions of years. And so these are some modern-day stromatolites
you can see in the water, they still form today. And here is a fossilized stromatolite, we can carbon date these and
determine how old they are. So we can date them back
to 3.5 billion years ago. And we know what this structure is
because of modern day stromatolites, that we know that these are
formed by bacteria biofilms. So evidence again that bacteria
were present on Earth in this time. We also have fossils of cyanobacteria, so this is a 3.5-billion-year-old
fossil found in Western Australia. I've got some other, additional pictures here. Here is a cyanobacteria from
today, a modern cyanobacteria, and you can see how similar the structure is to
some of these fossilized cyanobacteria as well. Now what about the effect of life on Earth? So we know that life doesn't just live on
the Earth, it actually changes the Earth. And you don't need to live in a better time to
understand this than current times where we are so grappling with the problems of climate
change as a result of human activity on Earth. But we're not the first organisms
to change the environment. The original bacteria on Earth were anaerobic,
so they didn't use oxygen for respiration. You know you, in order to have energy to do
anything, you need to be able to breath, right? But -- you need oxygen. But the original bacteria did not need oxygen. And bacteria were present on the Earth for
quite a long time, probably a billion years, before the first oxygen-producing
organisms showed up. And when they began producing oxygen, the oxygen was first absorbed
by minerals and other matters. So iron, for example, can be
oxidized to form iron-oxides. And the Earth was much like a
sponge, it absorbed the oxygen. So I should be clear that oxygen was
on Earth but not in the form of O2, the kind of oxygen gas that we breathe. So as these cyanobacteria began producing
oxygen, it was first absorbed by minerals. But eventually, the sponge was full as a -- thinking of the Earth as a sponge that
can absorb this atmospheric oxygen. Eventually, the sponge was full and the oxygen
began to increase in concentration in the air. And of course, I said earlier that most
of the organisms on Earth were anaerobes, so they were not able to survive
in the presence of all this oxygen. Oxygen is a very reactive molecule; it was actually poisonous to
most of the early organisms. So we refer to this as the
great oxygen catastrophe. Our first cells did not need,
or couldn't tolerate, oxygen. And when the cyanobacteria began
evolving and producing oxygen, many of these organisms were killed. Now some of the organisms adapted and they began
to be able to survive in the presence of oxygen, and even use oxygen as a way to
extract more energy out of carbon. And that is what we do as humans, we
actually require oxygen to get enough energy out of the nutrients that are in our diet. So after the production of
oxygen, you had this die-off, this sort of extinction even that occurred. Followed by a selection for organisms
that not just survived in the presence of oxygen but benefitted from oxygen. And you see the evolution of modern plants
and animals that exist on Earth today. All right. So that is a little tour through the
early organisms, the evidence that we have that there was life on Earth
about 3.5 billion years ago. We talked a little bit about
the speculation around how that might -- how life might have shown up. And next, we're going to talk about
a very well-established theory about how life changes on Earth over time.