CARTA: Climate and Evolution: Peter deMenocal:African Climate Change and Human Evolution

0
110

– [Presenter] This UCSD-TV program is presented by University of California Television. Like what you learn? Visit our website or follow us on Facebook and Twitter to keep up with the latest programs. ♪ [music] ♪ We are the paradoxical ape; bipedal, naked, large brain. Long the master of fire, tools and language, but still trying to understand ourselves. Aware that death is inevitable, yet field with optimism. We grow up slowly. We hand down knowledge. We empathize and deceive. We shape the future from our shared understanding of the past. CARTA brings together experts from diverse disciplines to exchange insights on who we are and how we got here. An exploration made possible by the generosity of humans like you. ♪ [music] ♪ – [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. ♪ [music] ♪.

NO COMMENTS