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We are the paradoxical ape; bipedal, naked,
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- [Peter] So I've been spending sometime working on this question about the role of
climate change in early human evolution. I think the best way to frame it is that our
best understanding of this problem is really it's this relationship between
climate and life. That is how has climate shaped life and how has life shaped
climate. I think the best way to convince ourselves that this is something that's in
our blood, in our entire world is to look at a seasonal map of primary productivity
in the oceans and on land over a multiple year period. You can see the waxing and
the waning of growth on the planet following the seasons. The planet
literally breaths with life that's paced by just the tilt of our Earth with respect
to the plain of the. . .of it's orbit around the sun.
Also if we take the longest possible perspective, we also find that climate
shapes life. Each of the five major mass extinctions in life on Earth over the last
500 plus million years, each of those five has been associated with a change in the
environment. Each one of these red lines here are where these mass extinctions
occur where-- somewhere between 50 and 90% of all species that were alive at the time
became extinct. Some would argue that there should be another red line there for
today.
So the last extinction was the extinction of the dinosaurs and that paved the way
for the rise of mammals, and today I'm going to be talking about this last little
sliver of time that includes our history. So if we look at this, this history of
human evolution. This is a greatly simplified diagram of the human phylogeny
or the family tree, and you can basically see these sort of red, yellow and green
sections. The human family tree has sometimes been liken to a Y-shaped pattern
or a more bushy one if you're more of a-- not so much of a lumper.
So we move from a single very long-lived lineage of Australopithecines up until
about somewhere around three million years ago where the family tree takes a branch.
One lineage, the yellow which sometimes referred to as Paranthropus. They are sort
of the line backers of the human family tree. They ultimately were not successful.
Then the blue lineage which is our own. That's the genus homo. It's been
described-- actually Rick Potts and I were part of a national research council
committee to evaluate these times of human evolution, these main times of human
evolution and appeared there are two main periods where there's a lot of action
going on, focused intervals of time. These include this time interval from about
three million to about 2.6 million years ago when there are at least several things
happening at this time, the extinction of Australopithecus afarensis otherwise known
as Lucy. Also the appearance of these two new lineages our genus homo and the genus
Paranthropus , and this is also when the first stone tools appears sometime around
three million years ago.
Then another event occurred sometime just after two million years ago where we see
the appearance of-- or the extinction of early homo, the emergence of homoerectus.
The first time we start seeing the Acheulian tool kit which is a much more
sophisticated tool kit that would become the model for future stone tools, and then
this is also the first out of Africa. The first time our ancestors left the African
continent, it was about 1.8 million years ago.
The point of my discussion today is to ask this question, "If climate is shaping
life, did it also shape human origins?" The question here really is not so much
just asking this question, "Did climate shape life?" Did it shape human evolution.
But really we have to ask the primary question which is "How did African climate
change?" That turned out to be a really challenging
problem for a number of very reasonable reasons.
So I'll let you know that the aim of this talk is really to illuminate these two
main ways that African climate has changed in the past. One is that, there have been
these very regular pendulum swings, if you will, very rapid in geologic time roughly
every 20,000 years where African climate oscelated between wet and dry conditions,
and it's been like a pace maker throughout millions and millions of years. The other
thing that's happened, super imposed on these wet/dry cycles is this long term
shift toward more open, arid conditions. So if you allow your mind's eye to
envision East Africa right now, you're probably thinking about the Serengeti or
these open fields of grasses. That ecosystem is actually a geologically very
recent phenomenon, only a couple of million years old.
So when we look at the past African climate change, we see that variations in
the strength of the African monsoon have been paced by variations in orbital
procession. This is basically that the Earth has a wobble, wobble in it's orbit.
Our northern hemisphere, our star Polaris right now-- The orbiting star 10,000 years
ago was actually the star Vega which is about 47 and a half degrees off in the
other direction. The Earth sweeps out-- The Earth's rotation actually sweeps out a
cone in the celestial sphere on a 20,000-year beat. What that does on Earth
is it changes the seasonal distribution of sunlight such that over Africa, the
northern hemisphere summers would have been about 7% more sunlight, and the
winters would have exactly the amount less. What this means for Africa is that
it strengthens, invigorates the African monsoon with this 20,000-year beat. It
acts really like a volume knob on the strength of the monsoon. So these
intervals in green that are shown here would be times of what we call an African
humid period of time when it would have been wetter in the past. You'll see that
they would been paced at this 20-year beat throughout this time period.
Now we know this is actually true, so this is actually predicted from theory and
what's amazing-- and this is one of the reasons why I work in this field is that
when we actually look at the geologic record for this, it shows that there were
these wet periods in the time. This is actually from a sediment core in the
eastern Mediterranean where a sediment core was drilled in this location. This is
a 10-meter core. This is one and a half meters then it connects with the top up
there. The base of this connects to top of there and so on. You can see that it has
these black and white layers. The black layers are these organic rich sediments
that accumulated when the Nile river outflow was much, much greater than today.
So that made for anoxic, or low oxygen conditions in the bottom of the
Mediterranean. So just like if you turn off the bubbler on your fish tank, all the
fish die and the organic matter goes to the bottom. That's what happened during
each one of these events. It happens not only for this time period here
representing maybe 200,000 years of time, but actually for millions of years.
This is actually a record, this is actually an outcrop that we see in Sicily.
It's actually a shot from a bar in Sicily, and these are actually people. These are
actually people working on this outcrop. You can barely see it that little red dot,
and each of these dark layers is one of these sapropel units. You can see how
they're bundled into groups of four and five which is this eccentricity modulation
of these processional cycles. Each one of these dark layers would be at 20,000 years
apart. This represents one million years of time going back 10 million years ago.
So this has just been a heart bit of African climate change going on for
millions and millions of years unbroken.
So if we look at Lake Turkana for example, Lake Turkana in Northern Kenya is nestled
in a desert environment today, but if you go to the shores of Lake Turkana you'll
see these bathtub rings where Lake Turkana was 50 meters or a 150 feet higher than it
is today during one of these wet phases 10,000 years ago. That's just one of these
little bars at the very top. You can see there have been just hundreds of these in
the past. So African climate has been nothing but continuously varying between
wet and dry, and wet and dry. So we look at the grassland expansion,
this is the second way that African climate has changed over the time scale of
early human evolution. Again, I'll show you this image if this grasslands from
East Africa. This actually cover something between 80 and 90% of the East African
landscape today. This envision of, let's say, of the Serengeti, this vision that we
all have of East Africa is actually a very recent geological development. It's only
about two to three million years old. What's interesting about the Savanna
grasslands is that they represent a very specific ecological adaptation to a very
specific environment. That environment is hot, dry, very seasonal rainfall and low
carbon dioxide. In fact the photosynthetic pathway that-- this is called C4 because
it represents the four carbon compound that's developed through photosynthesis
versus the three carbon compound. The C4 molecules or the C4 photosynthesis is
adapted to very high temperature and low atmospheric CO2 and generally dry
conditions. So these constitute the tropical grasses that dominate Africa but
particularly East Africa today.
They're there because the environment is very harsh. It's dry and seasonally moist,
only moist seasonally and it's also during low CO2 environments. So how can we
reconstruct how vegetation has changed in the past? Well the first thing that you
might say is, "Why don't we just look at pollen?" The problem with working with
pollen is that pollen's often not very well preserved in sediments. Any kind of
oxygenating environment, depositional environment, the pollen will disappear. So
we have a much more elegant way to explore it, which actually exploits the way that
photo-- the biochemical way in which the photosynthesis works using the C4 pathway.
So this is a brief overview of carbon isotopes. These are the two stable
isotopes of carbon. Carbon-14 is the radioactive or unstable
isotope of carbon. Carbon12, most of our bodies are made up of that 90-- 99% of
our-- carbon in our bodies is carbon-12. This is the most common isotopes of
carbon. Then carbon-13 is only about one in 100, about 1% of the carbon in the
environment. What we do is we measure the ratio of carbon-13 to carbon-12 using a
mass spectrometer and then express that ratio as a change relative to a standard
using this equation. This is the only equation I'll show for the day, but this
notation that we'll be talking about. So what we see here is that the C3 versus C4
plant signatures, the grasslands have a much more enriched or more positive C4,
carbon isotope value than do C3 plants. C3 plants would be trees and everything else,
and C4 plants in the tropics are going to be these Savanna grasses.
If we jump forward to a really heroic collaboration that existed with Naomi
Levin and Terry Cerling over the past decade or so, they've measured carbon
isotope values from soil carbonates which are these little carbonate nodules that
form in soils that record the photosynthetic pathway of the plants that
are above them. So you go to a geologic outcrop, you collect these soil carbonates
and you measure their soil carbonate. That tells you what the vegetation was at the
time that thing that formed. What you can see is this transition from lower values
to higher values, meaning an increase in the grasslands. This transition is almost
about a 50% increase in grass cover, and you can see that the time in that
transition is sometime around-- somewhere between three and two million yeas ago.
Compare that to this record of the sapropels where you get very rapid cycling
between wet and dry conditions. So this is a secular change. This is a very rapid
back and forth kind of like a pendulum change. So the way that we can now explore
past vegetation changes, it's actually very beautiful forensic tool that was
developed which allows us to look at fossils, but these are not megascopic
fossils. These are not fossil skulls, they're not
fossil shells, they're not fossil paleosol carbonate nodules. These are actually
fossil molecules. The molecules derived from plants, from ancient plants. All
higher plants have epicuticular waxes, that is you waxes on the outer surface of
the leaves. You've seen a rubber plant? That's nothing but wax on the outside.
Those waxy molecules are very robust. They last for a very long time. So we can take
a sediment. We can basically crush it and then put it in something that looks for
all the world like an espresso maker but it has very nasty solvents inside of it.
We'll extract the straw colored liquid which is loaded with these plant wax
molecules. Loaded meaning one in a billion of the carbon molecules in here will be
plant waxes. So they're not really loaded but they're sort of loaded. When we look
at them, we can actually measure with a chromatography what the compounds are, and
when you see these interdigitation between low and high peaks, that just shouts at
you. It looks like a hand and it says, "I am from a plant." This class of
molecules, this class of compounds are all derived from photosynthesis.
So what we can do is then we can take each of these peaks and instead of blowing that
carbon out into the atmosphere, we then direct it into a mass spectrometer. We
measure the carbon ratio of those compounds, and that's how we can figure
out whether that fossil plant that made those leaf waxes was a C3 or C4 plant.
My student Sarah Feakins is now a tenured professor at USC, did this at an ocean
drilling programs site here in the Gulf of Aden. A place is very difficult to go to
today. This is the extent of Savanna grasses there today. This is her record
of-- basically a snapshot record from the site. You can see it shows that same
trend, that same increase in values indicating the emergence of the
grasslands. So if you can pair her record to Naomi
Levin's record, you see they both have this transition. This both sort of secular
transition toward greater grasslands. What's impressive is that these times,
these green periods are the same green periods of major events in human
evolution. You can see that these transitions toward an initial expansion in
grasslands after about three million years ago and then really their establishment at
around two million years ago is coincident with some changes that we're seeing in
early human evolution.
What for me was the biggest, the most exciting discovery in this relationship
between climate change and human evolution was what showed up in the proceedings of
the National Academy of Sciences about a year and a half ago by Terry Cerling but
also a number of co-authors contributed to this study. So these are the same records
that I showed earlier. This increase in the indicators for grasses some time after
three million-- reaching peak values around two million. So this is basically
the same figures that I showed earlier. What Terry did and his colleagues, what
they did was to measure the carbon isotopes of the tooth enamel in fossil
skulls. So they took the teeth and they analyzed the carbon isotopes of the teeth,
and you are what you eat. You would see this relationship. You can see that when
you look at early-- the ancestral humans or ancestral hominids, you can see they're
basically tracking the environment. If the environment changes, your chemistry
changes. So basically the hominids are just tracking the environment. You say,
"Okay, we're done." Right? It's working, I mean basically they're just following
the environment. You can also say, "People couldn't care less about the environment
because look, they're just tracking the environment." Watch what happens. The one
group that doesn't do that is us. These blue dots are carbon isotopic analysis of
early and late members of the genus homo. You can see that they are falling away
from their other lineage Paranthropus which they shared the landscape with. So
Paranthropus , their diet was mainly derived from grasses, from Savanna
grasses. Homo, early homo got-- had a more variable diet and they were able basically
to extract a flexible diet from an increasingly inflexible landscape.
So more to come soon, this is my post doc, Kevin Uno for those of you who want to see
some truly spectacular new results. I'm happy to talk about that. But really, Rick
Potts and Andy Cohen have been leaders because they've led a drilling program up
and down in the riff valley collecting more, and more, and more of these sediment
archives. So what I'm showing you is just a teaser for what they're about to do. So
thank you very much Charlie.
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