This is geology B with all of us trampled. the Eon in which we
live is called the phanerozoic, which comes from the ancient,
Greek word, for Visible life, the Eon starts with the
Cambrian, which began 541 million years ago, But in recent
decades, it has become increasingly clear from the
fossil record that there was visible life before the Cambrian
and complex life at that. So what caused it to emerge then
and what causes it to proliferate and diversify so
vigorously in the early Cambrian, Rachel Wood is
Professor of carbonate geoscience at the University of
Edinburgh. She and her team have uncovered
fossils that suggests that the fuse of the so-called Cambrian
explosion was lit in the ediacaran.
The geological period that preceded, the Cambrian Rachel
would welcome to geology B. Thank you very much.
How old is the oldest large fossil?
Well, the record is quite remarkable.
We think the origin of life was anywhere between about 3.6 and
3.8 billion years ago, but the oldest large fossils.
We have our in rocks, that are approximately between 570 and
575 million years old. And in fact, the oldest examples
are probably found In Canada, in Newfoundland.
What did these last large fossils actually?
Look like when they were quite unlike anything that we know
today, they were remarkably large, they could be up to about
a meter in size and they had a very, very unusual organization
often a fractal. In some cases a fractal
organization. If you can imagine some of them
were like giant sea pens almost like giant feathers.
They had a stem and a feather-like part but they were
routed onto the sea floor, probably associated with
microbial, mats and biofilms. When I evoke the image of sea
pens is not to say that they were remotely related to sea
pens but they looked rather like giant feathers will see pens on
the seafloor others lived. Horizontally on the sea floor.
Again with this fractal organization where they
preserved because they had hard body parts know these oldest.
Since we call them had no skeletons whatsoever.
If they did have some sort of skeleton, it might have been an
organic skeleton, perhaps of something, akin to collagen, or
some sort of gel that supported their tissues.
But they were entirely soft-bodied, what sort of rocks.
Do we find these fossils in? Well, the oldest examples are
found and what we call clastic rocks.
So these are shales and siltstones, in other words,
rocks that are dominated by tiny.
Grains of quartz and Clay, some of the most remarkable settings
where these earliest communities.
These earliest microfossil, communities are preserved in
situ now actually preserved by being buried very very rapidly
by turbidity currents. And these are very turbulent
flows that start in Shallow Waters but actually eventually
come to rest in very deep water. So we know that these
communities were living in remarkably deep bits of the
seafloor. This bit of juice, Equal time.
How can we get precise dates for the rocks, in which we find
these fossils? Well, the most powerful and the
most accurate method we have for dating age of fossils is to find
Ash beds that are associated with these Fossil Beds,
obviously before or after interleaved.
If you like, with these Fossil Beds and we extract a key
mineral called Zircon. And Zircon is very, very good
for measuring and isotopic ratio with Ray genic Isotopes,
particularly you Uranium, and then it's daughter LED.
So we can measure this ratio between radiogenic, uranium and
it's daughter LED and extract a pretty accurate date for when
these Ash beds full. And so we can really bracket
where these Ash beds occur, we can really bracket the age of
these communities. Presuming these Precambrian
fossil-bearing beds, mostly fairly rare an outcrop only in
certain places on the planet where do we find them then?
Well, what's Extraordinary is these beds with these
communities of soft-bodied microbiota.
They are remarkably Global so we find them in Canada, in North
America, Siberia, Australia. There are remarkable populations
and Namibia so they are really widespread far more widespread
than you would expect for the preservation of soft-bodied
forms. Can you speculate us to why they
might be so ubiquitous It is an area really active research
because how could something that might have had the consistency
of a jellyfish few. Like, how did that get preserved
in these sediments? Where they've simply wouldn't be
preserved today. So, there are several hypotheses
to evoke the fact that perhaps seawater at this time had very,
very different chemical constituents today.
So for example, one set of ideas proposes that we had elevated
amounts of silica dissolved in seawater and this Effectively
aided the preservation of these by auteurs.
There are other ideas that suggests that these microbial
films, which we know they're associated with.
Might have formed some sort of death mask that preserved, these
forms and indeed other hypotheses of voc, the
preservation of key clay, minerals that again, could have
been a mechanism by which the claimant was coated, these
fossils and then aided preservation.
But all these ideas really rely on On non uniformitarian
principals. And other words, the present is
not the key to the past, the past is a different place.
Things happened differently, there you're a field geologist,
so you actually go and hunt for these fossils in the field.
Where have you been to get the fossils that you've just been
discussing? Well, the air is, I've spent
most time in the last five years or so.
Have been a Biblia particularly in the southern Namibia, but
also South East Siberia. Yeah, and actually, when the
most remarkable Expeditions I've been on recently was to a very
remote part of South East Siberia of the udemy river.
It was an area that really hadn't been studied extensively
since the early 1980s and I was very privileged to go with a
joint Russian Chinese and UK Expedition, we were out in the
bush for five weeks with no connection to the outside world.
Living off the land, bear is pickled mushrooms.
Beautiful, delicious fresh water fish.
We lived like kings, but it was a very, very remote existence
enjoyable. But very challenging, you've
mentioned that there were probably special conditions that
prevailed at that time to enable these soft-bodied creatures to
be. So are preserved.
In the fossil record does that information or any other
information? You've been able to extract from
the rocks at that time. Give you some Clues as to what
it was in the environment that triggered the emergence of
complex life at that particular time.
Yes. I mean but this is a very
complex problem the way to try and understand why something was
preserved may or may not be related to why the animal if
they were animals, they may not have been animals.
These biota, why would they were there at all?
So you have to think about the prerequisites for complex life
and very much the flavor of research in the last 20 years,
or so has been assigned. Idea that we needed, sufficient
oxygen present dissolved in seawater.
That would allow the emergence of complex life and we know that
the origin of animals in particular, certainly requires
some amount of oxygen, exactly. How amount of oxygen is very
much debated because we now know that animals with very, very low
metabolism. So for example, sponges which
cannot move they have very few, if you You like metabolic
requirements. These need much much lower
amounts of dissolved oxygen. Then say a predator that
actively boobs and has a high-energy lifestyle so there
are all sorts of ideas and hypotheses out there that evoke
the the stepwise. If you like or perhaps episodic
injection of oxygen as being a key, requirement to allow
increasingly more complex life with increasingly more
metabolically demanding life, Dials in particular, animal
Lifestyles. Okay, so is the idea that we
started off with an anoxic or largely anoxic planet and
eventually in the late ediacaran, the dissolved oxygen
levels reached a critical point which enabled complex life to
emerge. Yes, that's it.
There isn't a huge amount of consensus for these key bits of
geological time, particularly the easier car, and the early
Cambrian. But If you ask people this
question maybe 20 years ago they would have suggested two
episodes of increased oxygenation.
One of the great oxidation event which is approximately two point
1 billion years ago and then another one, perhaps close to
the Precambrian Cambrian boundary and that was the
trigger perhaps for the Cambrian explosion.
But now, we don't see a narrative of a single increase
episodic increase. We see these pulses of
oxygenation coming and going. And these May well, Of started,
in the EG car and possibly even older these pulses of
oxygenation where the shallow sea floor became oxygenated but
then and knocks it Waters impinged again on the shallow
shelves. So in other words it wasn't a
stepwise incremental increase. We just have these episodes of
oxygenation coming and going but with periods of more dominant
and oxy ER in between the other theme that's really emerging is
that are A shallow Marine shelves and possibly the Deep
Waters as well were incredibly heterogenous at the same period
of time. We can go to Anna media.
For example and say well I shallow shall cease were
oxygenated at this time but going through exactly that same
interval elsewhere. So for example China in fact
they were anoxic. So in other words we have this
very very heterogeneous distribution of dissolved oxygen
in our seawater that time And this in fact, may be the key to
why the fossil records of these places are very, very different.
I'm curious how it is. You can reconstruct what the
oxygen level was in the ediacaran.
Yes, there are various geochemical processes and they
fool. Very simply into two categories.
One set, which are recording a global of station level.
So, these would be things like, uranium, Isotopes.
So we can interrogate it uranium Isotopes, particularly in
limestone's and dolomites, and these can be used to.
Then reconstruct the degree of the seafloor globally that was
covered with and toxic Waters. They're rather tricky because
you have to make all sorts of assumptions about the Earth
State and they also need modeling.
So those are our Global proxies but potentially more powerful
are the use of local processes. You can actually look at the
condition just in a very small area, maybe a basin, or even one
part of a sedimentary Basin and one of the key proxies used here
is called iron speciation. It's a very simple, aqueous
geochemical technique where you extract all the iron from a
rock. But you divide that iron into
different types of iron and broadly into the amount of
highly reactive ion and the total amount of iron and it's
this ratio then it allows you to Infer that, that sediment, was
deposited in the presence or absence of oxygen.
So, this is a powerful technique which we can use to interrogate
at a really fine scale within basins individual sections
within the basins. And then compare the local redox
Dynamics through time of what area.
So for example, the Biblia, the another area for example,
Siberia or South China, it's the oxygen.
The main driving factor in the environment you That affects the
evolution of Life at this point. I think it's likely to be one of
the key drivers but it's also likely not to be the only key
driver. We have other effects.
We have Global temperature, changes, climatic changes.
And another idea that is really gaining a lot of traction now,
is the availability of nutrients so phosphorus and nitrogen.
For example, these are key building blocks of life and we
also know that as well as having a very heterogeneous and very,
very dynamic. Like Oxygen history in our
shallow Marine seized during this time, we also have very
very heterogeneous nutrient input into the Seas and we know
from the modern that areas where there's a lot of nutrients,
particularly nutrients either coming off land or nutrients,
upwelling from cold Waters. So, for example, the benguela
current that hits the southern African Coast, this creates
enormous productivity of life. So we think that These things
couple together, climatic changes changes in oxygenation
and changes in the availability of nutrients.
These all play a role in determining the evolution of
these early complex ecosystems. So I'm wondering if it's
possible to tease apart. These environmental factors that
you talk about such as oxygen and the availability of
nutrients from internal biological factors that may have
been at play here in the emergence and subsequent
flourishing of Life. Yes.
These so-called intrinsic characteristics are very, very
important and they should be seen in parallel with these
attempts, to interrogate the physical chemical World, which
we can from geochemistry and sedimentology and so forth.
And there's very, very interesting work taking place on
trying to understand the evolution of Gene regulatory.
Networks conserved versus non conserve parts of the genome and
trying to put these A genetics into the context of evolution.
So yes, this is a very, very important part of trying to
understand the emergence of complex life.
Can we use some of these intrinsic considerations to
understand what led to the emergence of biomineralization
or hard body parts that are shells and skeletons?
Yes. So one of the key incredible
Innovations in the history of complex life is this rise of
skeleton. So the ability of life If to
produce to co-opt minerals that are already there in the
environment and create a skeleton, or a shell out of
those minerals, one really interesting thing that's
emerging now is when you look at the biological Affinity of these
earliest by mineralize has, which appeared in the ediacaran
about 550 million years ago. I should say these earliest
purported Animal by mineralize errs.
These were the beginning of this sort of root of the Cambrian
explosion. And then the number of Tim's
diversify the credibly in the Caribbean, both, in terms of the
mineralogy, is our co-opted the types of skeletons and its sheer
complexity of these skeletons. But one idea, is that these
skeletons appear independently in unrelated lineages and this
tells us that the biogeochemical Machinery.
If you like all the genetic Machinery used to create by
minerals is a conserved feature, long predating, the history of
metazoa Animals indeed. It may be common to all
eukaryotes and one of the prevalent ideas is that there
was this pre-existing Machinery? That was simply co-opted for
another function, co-opted for the function of producing
skeletons. So this is led to the idea that
something must have happened. Perhaps in the environment, and
one aspect of that, is that the selection to have a skeleton.
May have simply changed. In other words, the fitness
gained from having a skeleton. Might have increased coincident
with the appearance of all these skeletons.
And the key idea there is that we can invoke a rise in
predation pressure and the rising predation pressure.
Then gave added Fitness to animals.
That bore a protective skeleton. And maybe this is the trigger
for why all these independent groups appear to produce
skeletons independently. But broadly around the same
time, when we spoke earlier you mentioned the idea that the rise
in oxygen. Able to the Bayou to have a
higher metabolism, more energy, enabled them to move and
therefore to predate. And so the ability for one
animal to eat it, another was originally triggered by the
environment and then led to this evolutionary change.
When now, as you say trying to tease apart, these intrinsic
biological features with these extrinsic triggers.
So, this is this idea that is really prevalent and
evolutionary studies of the relationship between
environmental change. And in creating environmental
opportunity and how life responds to that in other words
in evil, usually Innovation. And this is a really constant
dialogue and it's very, very difficult to tease them apart
because they often go hand-in-hand and there are
multiple feedbacks. I mean, one of the remarkable
things about our Earth, the most remarkable thing about it is, as
far as we know, it is the only place in the universe that
Harbours complex life. In fact, any life at all, But
this life co-evolved with the planet and all the things that
go to support this life are intrinsic to the workings of our
planet. The carbon cycle, the sulfur
cycle, the nitrogen cycle, the regulation of climate, and so
forth. So teasing apart, chicken and
egg. In other words, what came first
did the environment create a permissive environment that
allowed life to respond, or did life, innovate, and in.
So doing Change the way, our biogeochemical cycles, operate,
and in so doing, create this extraordinary feedback on the
planet? This is getting to the heart of
what we're really interested in. And of course, this is the Hub
of many, many research programs. So does this mean that this
period of innovation, which perhaps originally, we thought
was in the Cambrian can be pushed back, largely to the late
Precambrian, and that we see evolutionary ancestors of Of
Cambrian life forms in the ediacaran?
Well, I'm certainly one of those that is convinced that we can
see the roots of these Innovations in the ediacaran.
Of course, we should always remember that the fossil record
that we have is only the very incomplete record.
And these fossils are certainly only recording the minimum age
of any of these groups and what is the origin of these groups?
And if we look at a molecular phylogeny is for example, The
Proposal is that many of these groups have really old Origins.
So for example, molecular phylogeny is suggest that the
origin of the animals themselves could be anywhere between eight
hundred million years ago to 600 million years ago.
We have no record of animal fossils then at all.
But this is what the phylogeny is tell us.
So, it should come as no surprise that we can
increasingly make some links between EJ car and animals.
And Cambrian animals. One example could be this
ediacaran animal called Nama Colossus, it's known from
Namibia Canada, South America, and security.
It'll goblet shape fossil it has a goblet shape skeleton, but
we've recently found some exceptionally preserved
individuals that have soft tissue and there's soft tissue.
Of course. It depends on how you interpret
the soft tissue. But there is a curious structure
inside this skeleton. That is preserved.
Preserved in Paris and it's a tube of pyrite.
And we believe the best explanation for this tubular
structure, which is a symmetric and slightly j-shaped, is that
it's a gut and we think it's a gut of a group known as the
loafer truckers. Oh, ER.
So this is a group that today represents about a third of all
Marine, invertebrates related to the mollusks, the clams, the
bracket pods, the bryozoa, which little tiny Colonial animals.
Mi. So we think here we have a link
between these Cambrian and the terminal edia Karim.
That's interesting. So from what you've said, it
sounds like there was more continuity between the ediacaran
and the Cambrian than we first thought and was there.
In fact, a mass extinction or some kind of dramatic
environmental event that actually did take place at the
ediacaran Cambrian boundary. Well, the nature of this
boundary is really hotly debated.
What are the issues? Was, is that many of these
boundaries, accessions, globally are incomplete.
Many of them that have the richest fossils don't have these
key, Ash beds to date. So this is a real problem for
dating the order of events and also correlating globally
between sections of a remote from each other.
So there's two schools of thought one is that yes, there
was an Extinction. We see it this time at
incredible - isotope Excursion in carbon which suggests there
was a profound. Found perturbation of the carbon
cycle. And some have suggested that
this coincides with large igneous Province magmatic
activity. So this is very similar to the
mass extinction events that we see later in the phanerozoic.
Many of them, but not all can be associated with these large
igneous provinces. These are provinces where you
have a huge amount of Continental magmatic production
over a very long period of time, millions of years and these
create Rapid and sustained global warming and offered this
global warming can lead to the ocean circulation, becoming
sluggish and noxious developing one school of thought, suggest
that we had an event like this, a mass extinction event.
So perhaps anoxia triggered by this volcanism that created an
extinction of the so-called ediacaran biota and in effect.
Paved the way for the flourishing, beginning of the
Cambrian, biota the other school of thought, It is that there
wasn't a mass extinction, there may have been perhaps a more
modest turnover and nonetheless, we can determine transitional
biota across this interval. Indeed, we can find skeletal
fossils, which were previously thought to be entirely lower
camera. We can now find them in Egypt.
Karan sediments, they are transitional what we call small
Shelly fossils. So, I slightly fall into the
second category. And I believe that there wasn't
a mass extinction, I may be proved wrong, but I think at the
moment, the evidence for mass extinction is not compelling but
rather we see a series of transitional by auteurs.
So in other words we see this slight turn over at the boundary
but we see many turnovers before that and we see turnovers
afterwards as well so it may be possible to reframe The
Narrative of the Cambrian explosion as perhaps Not being
Cambrian at all and simply being one of the several radiations of
complex. Life animal life, indeed.
That actually precedes the Cambrian explosion and the
Cambrian explosion is one of the final manifestations of this
event. However, having said that, what
seems to be very characteristic of the Cambrian explosion is
this genuine explosion of fossils of predators with?
I think that is a distinctive part of the Cambrian but the
Caribbean exposure Couldn't have happened without all these other
radiation of all these other transitional by auteurs before
that. So, does that suggest the view
that the Cambrian explosion, when it actually did take place
was more of a intrinsic biologically driven change.
Yes, it is a very interesting and fundamental question,
there's no doubt that ecological drivers were very important in
the Cambrian explosion. We know how powerful a driver is
of the rise of predators and we have this this phenomenon known
as an arms race. It's really form of evolutionary
escalation. In other words, a new Predator
revolves with new horrible mouth, parts, or grasping
appendages. And so the prey has to respond
by in some way acquiring some sort of either defensive
skeleton or change. Its Behavior, becoming
nocturnal, for example, or hiding in a crevice, this ends
up in this arms, race is a constant arms race.
So this is certainly important. But we also know that the lower
can be was still a time of very Dynamic Redux, can you explain
what you mean by redox? So by redox, I simply mean the
distribution of oxygen and I normally mean the distribution
of dissolved oxygen seawater. So in other words where we had
free oxygen, we can refer to that as a toxic conditions,
where there was no oxygen at all.
We can refer to that as anoxic conditions and all sorts of
other shifts in the carbon side. Other Cycles were ongoing at
this time. So I think again, we can't just
pin all this on one factor, all these factors are
interconnected, and I think this is what makes the ediacaran.
So phenomenally interesting is that we have to unpick all these
factors and really understand how they are interconnected and
understand the drivers and the feedbacks.
It's many, many lifetimes work. I'm struck by the fact that,
although, we've now pushed back the origin of complex life into
the ediacaran, a few tens of millions of years into the
Precambrian. That's not much in the grand
scheme of things. It still leaves well, over three
billion years of History, without complex life.
Can you speculate why? It took so long to arrive.
So, why it took so long for complex life to appear is
unresolved, and it is a tremendous source of current
research. One that pervasive ideas, we
have this phenomenon called the boring billion which is a
billion years when not much seemed to have happened or at
least, we thought not much happened.
So in other words Eeveelution, it just seemed to be the same
fossils from beginning to the end.
However, this record is really starting to be interrogating, a
lot more detail now and without doubt we will be revealing
exciting, new fossil finds. But also the geochemistry of
this interval is being interrogated and it's not boring
at all. The seem to be episodes of
oxygenation. In other words, this pattern
that we see in the other car and of these pulses of oxygenation,
maybe hand-in-hand with pulses of nutrients, that may well be a
motif that we can trace back into this boring billion.
So we're constantly pushing back the origin of these key building
blocks. If you like further back into
time. And I think that this boring
billing will prove anything but boring in the years to come So
if you were to speculate about EXO life or life on any other
planet, would you expect therefore that there are some
fundamental constants of nature? If you like that it really does
take billions of years for the appropriate complexity to
evolve. Whether it's what can be
permitted by the environmental factors or whether what is
needed in the molecular biology to biomass sufficient
complexity. Yes, this is a prevalent theme
that simply Life needs a very, very long time to assemble the
necessary genetic Machinery, the gene regulatory Frameworks and
so on. But of course, we only have one
example Earth, we cannot be run the experiment, and, of course,
we know that it's very, very difficult to Define life, Define
this quality, this elusive quality that we call life and
there are some wonderful definitions out there in terms
of self-regulation and homeostasis.
Yes, but we're in the midst of a pandemic, which is caused by a
virus. And many would argue a virus
isn't living, but look at the power of a virus.
Look at the power of something that is not living, but it has
this extraordinary ability to replicate by co-opting, human
genetic machinery and get better at it tremendously.
So it comes down to. I suppose our ideas about what
is life. What is complexity?
We place animals at the top of the tree of life, Because we're
one and we're big and we appear to be in control, but maybe we
should rethink the way we perceive the Tree of Life.
Rachel would thank you very much.
Thank you very much Oliver. It's been a pleasure for more
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