A mere 542 million years ago, the Earth's landscape was a barren wilderness.
4 billion years after the planet's creation, no animals lived on the land.
The most developed organisms were simple animals living in the oceans.
Something extraordinary was happening in the seas.
Over a period of 20 million years, at the beginning of a period called the Cambrian, an
explosion of new animals developed below the water's surface.
These later gave rise to most of the diversity of animal life that now exists on Earth.
But the cause [of] this sudden burst of life, which geologists call the cambrian explosion,
remains a mystery.
The Cambrian explosion was really Biology's big bang. Things went from moving very slowly along the sea floor without any
predation to suddenly all the different type of ecologies that we find today. Your life literally exploded.
What triggered the cambrian explosion? We don't really know. There are many different possibilities, different hypotheses
and we've got to remember [that] cause and effect are often difficult to disentangle.
So what happened exactly?
To try to understand it, you need to look a little bit farther back in the history of the Earth to the
Ediacaran period which preceded the Cambrian. It was during this time that the first large animals evolved,
but they were shell-less.
Before the Cambrian, we go into what's called the Ediacaran times.
And this is an extraordinary interval where we have the seafloor
populated by basically weird creatures. And quite frankly, we're not quite sure whether they're animals or something else entirely.
These are organisms which seem to have mostly survived on absorbing
nutrients from the seawater itself. They may not have had any gut. Some of them may be able to move.
These creatures are principally soft bodied, that is without a skeleton,
and then they seem to more or less disappear as the Cambrian explosion kicks off.
After 45 million years of slow evolution those weird creatures suddenly disappeared to be replaced by those of the Cambrian.
The first Echinoderms appeared 540 million years ago.
Starfish and sea urchins are their descendants.
Brachiopods followed after some ten million years.
The trilobite, the cambrian's most iconic fossil, joined the explosion towards its end. It showed up a few million years later.
There are a number of theories about what happened that enabled these creatures to appear.
There's a theory, a
hypothesis, suggesting that one of the factors that holding back the evolution of animal is
the availability of oxygen.
Photosynthesizers, things like cyanobacteria,
algae, plants, make oxygen and
organic carbon. And once the organic carbon is buried in the sediment, the oxygen is released in the
ocean and the atmosphere
for animals to use.
Prior to the Cambrian explosion
oxygen levels may have been too low for large active animals to survive,
but once those levels increased, larger animals were able to survive and proliferate.
Then animals burst and diversified in many many different forms.
Evidence to support this theory of higher oxygen levels is scarce, but there may be a link to a phenomenon called snowball Earth.
This was a long period of recurrent ice ages. When the glaciers melted huge amount of rock were exposed and eroded
pouring nutrients into the water that encouraged the growth of photosynthetic algae, which would have caused oxygen levels to rise.
But the last of the giant ice ages ended around 90 million years before the cambrian.
Things like oxygen levels which are fairly clearly increasing during this interval close to the cambrian explosion must have been important
but oxygen was accumulating already much, much before then.
Another theory for why the cambrian explosion occurred is that minerals became increasingly available in the oceans due to post glacial erosion.
These could have been used for the construction of shells, but
cambrian shells were made from a range of materials.
This suggests that they were the result of parallel evolution in different animal lines not a single innovation.
That means something encouraged them to develop on more than one occasion and
shells are costly to make. It's unlikely mere abundance of minerals would have been enough.
Could a transformative innovation in the animals themselves be the explanation?
One such innovation could be bilateral symmetry which concentrates nerves and sense organs at an animals front and
encourages the development of an efficient gut with a mouth and an anus.
We are inventing nervous systems or inventing eyes and other sensory systems.
We're learning how to move fast, how to swim effectively. So you have a whole cascade of feedbacks.
And it's really trying to pinpoint if you like the trigger versus all the other consequences which flowed from that initial point.
The cambrian explosion
happens after a series of events in my opinion.
First you need to have genetic building blocks
to make an animal. You also need oxygen to
support animals, to support animal metabolism. But that alone is
not sufficient either. You also need a mechanism
to drive the system into a runaway situation. You need to make the system work
very fast. And one of the mechanism to drive the system to a runaway
situation is
ecological feedback.
Ecological feedback, in this sense, refers to the interaction between predator and prey.
During the Cambrian, predators evolved new mechanisms such as vision to track down their lunch. A
good part of what's happening in the Cambrian explosion must be the motor of ecological change. And
one of the really big
important gear changes,
so to speak, to pursue that analogy, is in the way in which predators and prey
interact. Part of the explanation for the widespread development of skeletons must be the rise of predators.
And we see other evidence in terms of mouth parts, even gut content sometimes. You can see what one animal swallowed.
And all these things suggests that we are moving into an ecology which is radically different from anything ever before. An
integral part of this is the evolution of nervous systems. And nervous systems of course include ultimately brains and
ultimately of course when animals come onto land. At some times even in the oceans big brains which start thinking, "oh".
Some believe the emergence of vision during the cambrian period caused the rate of ecological change to accelerate. I
believe that vision must have played a significant role. I think the fact that that event
coincides with the start of the cambrian explosion
possibly could indicate that vision was actually the only thing that was required to trigger the cambrian explosion.
Light today is the most powerful stimulus of all and vision is the most powerful sense.
When that's unleashed on the world for the first time it's going to create chaos.
Now the capability to sense light
has been there for a long time. Even some bacteria have a way to sense light.
Many microorganisms have a way to sense light. But the very complex eyes,
they can very precisely
position the prey, they can view three dimensions and allow the predators to very effectively catch the prey.
That's a very important part of the ecological feedback that allows the cambrian
explosion to happen in a very rapid rate.
Now we have this pinpoint accuracy, a predator could spot a prey and
also
because it's got its fast swimming parts and hard part itself, it could get to that animal instantly and tear it apart.
So that means now there's huge selection pressures on all those soft bodies animals to evolve
defenses against this which means really hard parts. It means
shields and spines and also hard swimming parts to swim away itself.
But not everyone agrees.
Now there is absolutely no doubt that eyes did evolve as part of the cambrian explosion. My reservation with it
is that eyes come in all sorts of shapes and forms. And primitive eyes are really little more than what we call eye spots.
They're sensitive really to sort of shades of grey and white and no more than that.
And that's quite sufficient for the great majority of animals. They don't actually make an image on their eye. Whereas
I think one of the dangers, if I may say so, was seeing evolution through our eyes.
We have fantastically well adapted eyes which are the product of millions of years of evolution
And there's little doubt that those really sophisticated eyes only evolved a bit later.
There is one other possibility.
The disappearance of the Ediacaran creatures may be the cause, not the consequence of the cambrian explosion.
The explosion resembles the recovery from a mass extinction.
Signs of a catastrophe 542 million years ago could easily have vanished by now, but that doesn't mean one didn't happen.
Perhaps the bilateral ancestors of today's animals were simply hiding in the shadows of the ediacaran world until opportunity knocked.
Whatever actually did happen though, they took that opportunity with momentous consequences for the future.
Humans are direct descendants of the cambrian explosion.
we have primitive fish
From deposits such as a burgess shale in Canada which we can really say are our ancestors.
From the human point of view, the true significance of the cambrian explosion is that Homo Sapiens wouldn't be here if it hadn't happened.