[BLANK_AUDIO] There have been many remarkable
transitions and Changes in the history of life on Earth. Well, one of them that we're going to Talk about here is the rise of
multicellularity. What do we mean by a multicellular
organism? Well, we mean an organism where The cells have differentiated into
specialized functions. For example, we are multicellular
organisms. We have cells that do bone structures. We have cells that are skin cells. We have cells that form the liver and the
heart. All the cells do different things, but
they all Communicate with each other in the
totality of the organism. A multicellular organism is an organism
that contains Cells that are specialized and carry out
particular functions. But all communicate to create a total
organism. This is an unusual way of doing life. For about three billion years, all life on
Earth was microbial, unicellular. Single-celled organisms forming the
stromatolites that we saw earlier. All these microbial mats, just collections
of undifferentiated Microorganisms simply reproducing and
producing the same cells as They grow in particular environments.
The controversial evidence for These very early life forms is in the Fossil record, and we've talked about that
already. Also chemical signatures. But all of this evidence, and it gets Better as time goes on through the history
of Life on Earth, gives us this knowledge
that for About 3 billion years, life just continued
being unicellular. And then something, remarkable happened.
A complete change. Between about 585 million Years ago and 542 million years ago, the
Ediacaran Fauna, Which are fossils in the rock record, that
suggest the first multicellular organisms.
And these fossils are very enigmatic. We don't really understand them.
Some of them have strange frome shapes. Some of them have strange tubular shapes.
They seem to be experiments. Experiments in early body plans as Cells began to differentiate, become more
specialized, and form larger Complex organisms that would ultimately
lead to the multicellular life That we know on Earth today.
They were first discovered in Australia. This is an artist's impression of these
fauna, As its been found in the rock record. And what they might have looked like in
these early environments over 500 million Years ago. And you can see these strange Three-dimensional shapes that suggest
these early experiments In different body plans, as life began to
emerge from its unicellular state. To complex multicellular organisms with
specialized body plans And cells forming specialized parts of the
different organisms. So an obvious question for astrobiologists
is, How did this multicellularity arise and
why did it arise? Well, it still remains controversial, and
in Fact the rise of multicellularity is one Of the great puzzles in astrobiology, and
one of the great puzzles for biologists. Here are some possible examples of ways in
which this might have occurred. There may have been some sort of genetic
change. At some point in the early history of Life, in the early history of those
multicellular Organisms, the genetic material of DNA DNA
changed. And there was mutation that led to
specialization of cells. So what were unicellular, non-specialized
cells, the genetic information changed. They mutated, and some cells became
specialized to do one function. And some cells carried on doing Another function, but they remained in
communication. So they remained in a complete organism. Well, why would this occur in the first
place? We don't really understand.
Maybe it was something to do with the
complexity of genetic material. Genetic information that triggered at some
particular point in the history of life, This complexity in the genetic code that Allowed for this differentiation to have
occurred. Another possibility is not just changes
internally in the genetic information But also changes in the environment. For example, the rise of oxygen might have
been important for multicellular life. Why would that have been the case? Well, once we have oxygen in the
atmosphere, we can do aerobic respiration. That's the way in which you and I make
energy. We essentially burn organic carbon, for
example, in Our sandwiches, using oxygen in the
Earth's atmosphere. And that releases a tremendous amount Of energy. Energy that can be used to run, to jump,
and even to power the human brain. And that's the same for other
multicellular organisms, as well. Without oxygen, the sorts of metabolisms,
the sorts of ways in Which life can get energy, are much less
energy-efficient than aerobic respiration. So, the rise in oxygen in the Earth's
atmosphere might Have been one of the triggers that allowed
for complex organisms, Multicellular organisms to start to arise
on the surface of the Earth. Of course it's possible that none of these
things was mutually exclusive. It might be that a number of these factors
occurred at the same time. The rise in oxygen might have Allowed for aerobic respiration, complex
multicellular organisms. And that might also have helped trigger
genetic Changes within organisms that allowed them
to exploit these New potentials. So perhaps a number of things contributed
to The rise of multicellular life that were
interconnected. But Once multicellular life did arise, why was
it selected for? Why did it persist? Well, there are a number of possibilities.
First of all, once organisms became
multicellular, they became larger. And some sort of arms race may have
occurred between organisms. Larger organisms would have evolved to eat
smaller organisms. Then there would have been a selection
pressure, a Darwinian Selection pressure for organisms to get
larger, to eat those new Predators, and so on and so forth. An arms race becomes established as
organisms become larger and more Complex to deal with the new biological
environment in which they're living. Cellular specialization can lead to
increases in efficiency. Some cells being specialized for
locomotion, other cells being Specialized to generate energy, and so on
and so forth. This specialization, this division Of labor in the way in which cells
operated might Itself have increased the survival Potential of life in particular
environments. Multicellular organisms may also have been
able To develop better physical protection from
the environment. You can think about things like shells and
skeletons, that allow life to To move around, to protect itself From physical extremes, to escape physical
extremes. So multicellularity Would have led to would have led To advantages in living in the natural
environment. And finally, computational advantages. As you get more complex organisms, as they Become multicellular, you can get more
complex behaviors. You can appreciate the behavior of a dog
is More complex than the behavior of a
microbe, for instance. So multicellular organisms can do things
like run Away from physical stresses, run away from
dangers in The environment. These computational advantages that allow
life to live in a Complex world, and increases its, increase
its chances of survival. All of these things would have allowed
multicellular
Life to have been successful on the Earth. And once it had emerged, selection Pressures, evolutionary selection
pressures would have Allowed that multicellular life to
persist, and give rise to the diversity Of multicellular life that we see on the
Earth today. We know that at about 542 million years Ago, this complex multicellular life was
well established. How do we know that? Well, as life became more complex, as
multicellular life began to diversify, Some of these creatures began to produce
skeletons, bones, and also shells. And bones and shells are much more easy To preserve in the rock record than soft
tissues. So, complex multicellular life with
skeletons and Shells is well preserved in the rock
record. And begins to give us the first
unequivocal evidence that complex Multicellular life had begun to emerge on
the Earth at about 540 million years ago. And this period, where we start to start
to see these Multicellular organisms rapidly increasing
in the Rock record, is called the Cambrian
Explosion. Literally because of an explosion, a vast
increase in The number of preserved multicellular
organisms in that rock record. It would be very easy to think of The history of life as a very simplified
thing. We start off with unicellular organisms,
and then the emergence Of multicellular life, and a gradual
increase in the complexity and Diversity of that multicellular life.
But that wouldn't be true. In fact, we look in the rock record, We can see that there have been mass
extinctions. Periods in Earth history where large
percentages Of these organisms have literally been
laid waste. In the rock record, we notice five mass
extinctions. And some people even think our current
time, because of the Scale of human impact on the biosphere, is
a sixth mass extinction. During these periods of mass extinction,
diversity
Is reduced but then picks up again
afterwards. In fact, interestingly after many of these
mass Extinctions, diversity has increased after
these extinction events. What has caused them? We'll, they're very controversial. For example the cretaceous paleogene
extinction 65 Million years ago, when the dinosaurs went Extinct, is thought to have been caused by
an asteroid or comet impact on The surface of the Earth. That asteroid or comet impact would have Lofted dust into the atmosphere, shutting
out sunlight. When the sunlight shut down, it would have
prevented photosynthesis. Plant life would have died off removing
the source of food For many organisms, multicellular
organisms on the surface of the Earth. And as a result, food webs would have Crashed, and many organisms would have
gone extinct. Other mass extinctions might Have been caused by massive volcanoes,
changes in the chemistry Of the oceans, changes in the chemistry of
the atmosphere. So there are many mechanisms by which mass
extinctions might have been caused. And it's one of the go-, the great
interesting Areas of astrobiology is to try and
unravel the Causes of these mass extinctions by
looking at the Geological, geochemical, and environmental
changes that might have occurred. That are recorded in the rock record, from
which we might Be able to ascertain what caused these
mass extinctions in the past. But we should also remember that Mass extinctions have created
opportunities for life. The death of the dinosaurs 65 million
years ago Undoubtedly opened the way for the rise of
mammals. And eventually to us, and eventually the
rise of human Intelligence, and the intelligence on the
surface of the Earth. So mass extinctions can change the course
of biology. And also create new potentials and new
possibilities
For the emergence of new new forms of
life. So, what have we learned in this lecture. Well, we've learned that for the first 3
billion years Of history, life on Earth, life was
essentially unicellular, microbial. And then, we begin to see, about 580
million years Ago, the emergence of the first Multicellular organisms in the rock
record. Strange, enigmatic shapes of organisms
that Suggest early experiments in body plans. We don't fully understand why this
happened. It could've been changes in the genetics,
the genetic information of organisms. It could've been changes in the
environment, the rise of oxygen. It could've been multiple reasons that
came together and created The right time, the right conditions for
multicellular life to arise. We've looked at some of the advantages Of multicellularity, size and
specialization that would've been Very successful traits for organisms
trying to make A living on a planet with many challenges. And we saw that in the rock record about
530, 540 million years ago, a sudden sharp rise
in the preservation Of multicellular organisms. Because of hard body parts that were
preserved in The rocks and give us an indication that
multicellular Life was well on its way to becoming one Of the dominant forms of life on the
planet. And we've also seen how this rise Of multicellular life has not been
continuous. It's been punctuated by mass extinctions
that Have wiped out life, percentages of this
life. But after these mass extinctions, The diversity of life has continued to Increase, leading to our present day
biosphere. It's even thought that our present
activity in The biosphere might be another type of
extinction event.
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