EEB 122: Principles of Evolution, Ecology and Behavior?

18. Major Events in the Geological Theatre

https://oyc.yale.edu/ecology-and-evolutionary-biology/eeb-122/lecture-18

Now today we're going to be talking about some of the major events in the geological theater. This is the second of three ways that we're looking at the history of life. The first was rather abstract; it had to do with major transitions and with reorganization of genetic information, units of selection, things like that. Today we're going to talk about how life shaped the planet and how the planet shaped life. So this is a quick run through a 4.5 billion year process.?

• At the beginning, we had a reducing atmosphere, and the source of O2 was photosynthetic bacteria.?Once?chemosynthetic?as well as photosynthetic?bacteria get going and start producing a lot of oxygen, there's a tremendous mass of stuff on the face of the earth that has to be oxygenated before there's any free oxygen.?Until about half of the age of the planet, the concentration of oxygen in the atmosphere was less than 0.4%.

• And the evidence that we have of when there was free oxygen in the atmosphere is essentially the age of the iron mines of the world.?There was ferrous oxide--it can dissolve in water--floating around in the ocean, and when the oxygen level of the atmosphere got high enough, it oxidized to ferric oxide, and the ferric oxide fell out of solution, and when it fell out of solution it made the iron mines of the world. That hAppened 2.3 billion years ago.

• This kind of process continued with other sorts of elements. So we have copper coming out at about 1.7 billion years, at a higher concentration of oxygen. And the consequences of free oxygen are that an ozone layer forms in the atmosphere. That screens ultraviolet light and that drops the mutation rate, and it's probably only because the mutation rate dropped significantly with an ozone layer that we could evolve large long-lived organisms.

• Once you have oxygen in the atmosphere, you can start getting nitrates. Nitrates are oxygenated nitrogen. So you won't really have nitrogen fertilizer until you have free oxygen, and that then also became a key nutrient for algae. So there's a whole sequence of important chemistry that goes on over a period of about 3 billion years.

In our early environment, the sun was only about 70% as hot as it is now, and by about 500 million year ago it was up to 95%. The early environment of the earth was a meteorite bombardment.?The heat flow out of the molten mass forming the core of the earth has tended to drop off and stabilize. So it has a continuous radioactive input, but the original heat from the entire planet being molten has gradually radiated. So we're stabilizing at about the heat flow from the radioactivity in the earth.?

The continents formed and stabilized at about 1.8 to 2 billion years ago, and these things called major orogenies are major chunks of continent coming up and major mountain ranges getting built. Now the collision of plate tectonics has continued to form mountain ranges since then, but this just stabilizing the continental crust took about 2 billion years.

At the origin, the CO2 level was?much higher. The atmosphere was more than what we would call 100% CO2, because it was thicker at that point.?This has dropped down to about 3*10^-4?atmospheric pressure for CO2. It's actually a small component.?

Oxygen rose and probably reached present levels at about 5 or 600 million years ago.?At about 27%, wood will catch on fire spontaneously, at current atmosphere pressures.?At the beginning, we had water, hydrogen, carbon monoxide, lots of steam; a lot of that escaped to space. There were meteorite impacts. The CO2 curve has gone down. The oxygen curve has done up; there's some indications it's gone up stepwise.?

Temperature, we don't really know accurately what the temperature was back before 3.5 billion years, but we can be pretty sure that at and after the origin of life, water was liquid on the surface of the planet; so that sets an upper limit of 100 degrees Centigrade. And temperature has gone up and down in a number of cycles over a fairly long period, and there have been some major Ice Ages.?

How do you recover that??

• One of the ways that you can do it is you can look, if you have leaves of fossil plants--so if you've already got plants that evolved and they have leaves; so maybe about the last 300 million years or so--you can look at the stomatal ratios on them.

• It has been calibrated. Plants have to make more holes in their leaves?if there's less carbon, they have to have a bigger mouth so that they can feed more efficiently. And they can have fewer holes in their leaves if--and they can be smaller--if there's more carbon in the atmosphere. Basically this allows you to plot and estimate a curve.

• There was massive withdrawal of carbon dioxide from the atmosphere from the Ordovician?through the Permian.?And then there was a re-injection?going into the Triassic?and then there's been a more gradual withdrawal down to the current level.

And if we look at where the carbon dioxide went, a lot of it got locked up in limestone, in sedimentary rock. Then a lot of it is in organic carbon. A lot of it is in the ocean, is bicarbonate. These are by far the largest sinks, but there's a lot of bicarbonate ion in the ocean. This is all the fossil fuel on the planet right here; so this is all the coal and oil.?

So the carbon balance of the planet is extremely dependent upon what hAppens in rocks, and that if there are small geological changes in the cycle of how carbon is going in and out of rock, and whether it's being subducted as plate tectonics proceeds or not, is going to make a much bigger difference to the amount of carbon in the atmosphere than the amount of fossil fuel that's being burned, or the tree cover of the planet in forests, which would be the living biomass term down here.?

If we look at the way that life structures the planet, one of the very important things that life has done is that it's made soil. And we don't really start to get soil, which is a big complicated piece, an engineered niche that plants create, until we get complicated plants on land.?

• The first ones on land are probably things like liverworts, and our first fossils are club mosses, and that's hAppening back at around 400 to 500 million years ago.

• There are fossil soils that?have roots in them, and those roots suggest that the first time that there were real trees was at about 350 to 400 million years ago.?This is relatively recent, in terms of the age of the planet.

• So we get really modern soils with layering, and with evidence of seed plants in the Carboniferous. So that is the age at which most of the coal mines of the earth were laid down; it's about 300 million years ago.?

Bacteria,?the?archaea and the eubacteria,?engineered the planet?and?continued to do so.?

• They are the ones that play a huge role in the carbon cycle. They're producing and oxygenating methane. They're fixing carbon dioxide.

• In the nitrogen cycle, the bacteria are fixing nitrogen from the atmosphere; they fix it as ammonia. They oxygenate ammonia to nitrate; they de-nitrify nitrates to ammonia.

• The nitrogen in all of the proteins on the planet is essentially originating through bacterial processes. So that's how it's getting from the abiotic world into the living world.

• There are sulfur bacteria that are extremely ancient, and which evolved in an environment in which much of the energy coming into living systems was coming from things like sulfur, rather than from sunlight, and they oxidize hydrogen sulfide to sulfate; they reduce sulfate to hydrogen sulfide.

• Iron bacteria are converting ferrous to ferric iron, and they're influencing a degradation of manganese and copper deposits. It?goes?on at spreading centers at mid-ocean ridges, or it is going on where there is heat flow which is taking ocean water through the ocean crust. When they do these reactions,?they leave a metal deposit behind; which is why the floor of the Pacific Ocean is covered with manganese nodules.

Those are all aspects of how life has modified the planet. How has the planet modified life? ?One is through continental drift; another is glaciation; mass extinction; and then local catastrophes.?

• Continental drift and mass extinctions are both out there at the scale of hundreds of millions of years.

• Glaciation has two scales. There've been at least three times when it's been really quite cold. But within those longer periods that are cold, the glaciers have come and gone many times. So the North American glaciation lasted 2.5 million years, and the glaciers came and went about 15 times, in North America.

• The local catastrophes, it all depends on which particular kind that is. They occur at different time scales. The point of all of this is that often the past configuration of the planet, whether it's the location of the continents, or the temperature of the earth, or whether you could expect to live in a secure environment, have at times been extremely different from what we currently see.

Here's the illustration of the?last 400 million years of continental drift. And?people are producing models that can now take this back to about oh a billion years.?

• This is Gondwana. So Pangaea was a little bit earlier than this; that was when all of the continents were together. South America,?Africa,?Antarctica,?Australia and India?stuck together for awhile, before they came apart.

• When Gondwana split up, it had some things living on it. The ratite birds, and they are flightless and they don't swim, and essentially they got rafted around on pieces of rock. And you can lay a molecular phylogeny of the ratites onto these continents and it just ties them right together.

• There's another thing that hAppened with the breakup of Pangaea. Laurasia went north, Gondwana went south. In between, for awhile, there was a thing called the Tethys Sea. And this is the configuration of the continents about 50 million years ago, in the Eocene.

• There was a warm kind of Mediterranean coastline that stretched from eastern North America, through Nepal, into what is now eastern China. This was before India rafted north and Africa came north and closed off South Asia.

This have accounted for some of the similarities in the plants that you find in the Appalachian Mountains and in China.?The?rhododendrons, viburnum; there are a number of tree species that share a phylogenetic relationship across that huge geographical distance, and it's thought to have been the signature of a corridor along which seeds could move 50 million years ago.?

Now how about glaciers? This is the Phanerozoic; the Phanerozoic is the term for everything that's hAppened since the Cambrian started.?There was an Ordovician Ice Age and a Permian Ice Age, and then there was an Ice Age just in the Pleistocene. There are signatures in the rocks to tell you what latitude you're at?and how cold it was. These are usually in the form of isotope ratios for oxygen and carbon and stuff like that.?

In the Permian, Gondwana is still together. It breaks up at about 225 million years ago; somewhere between 225 and 250. The Permian is at 250 or?251. And there was a southern ice cap that was actually connected, and actually these continents were all together; and you can see from the arrows the direction in which the ice was flowing. You can find rocks, from Africa, that were scraped off by the glaciers and deposited in Brazil.?

• The climate since then has actually mostly been warm.?If you look on this set of maps--this is 50 million years ago; 35 million years ago; 15 million years ago; middle of the Pleistocene, about 1.5 million years ago; and very close to today, mid-Holocene would be say about 5000 years ago--and you look at how much of the planet is temperate and tropical, look at how tropical the Eocene was.

• That was all tropical rainforest, and in?the Oligocene?there?was still a huge area of tropics, and the Miocene still had pretty good tropics. But at the last glacial maximums, the tropical rainforests were reduced to a few patches. We're living today in a relatively cold, relatively dry world.

• Scandinavia and Northern England are completely under ice, as is the North Sea. The Sahara Desert was humid. You can go into the middle of the Sahara Desert and you can see rock paintings that humans made there, where they're recording hippopotamuses and things like that, living in the middle of the Sahara, at this time. And the major tropical forests shrank.

It?is more or less the global pattern. The grey now is ice. The green is grassland; the orange is rainforest. So there are tropical forest refugia, in certain places.?

If you were to go into the south, what is now the South China Sea, which is currently covered by water, elephants and tigers could walk out, over that, because it was dry land--enough water had been tied up in the ice to drop the sea level down that much--and that is how they got to Borneo. So they could actually just move down from Asia and get out as far as Borneo, but they couldn't make it across Wallace's Line--there is a deep-water passage there that Alfred Russel Wallace documented in the biogeography of Indonesia--and they couldn't make it to Australia or New Guinea.

So the sea level has gone up and down, and that's changed continental margins and the ability of things to move around in them. So that's impact of glaciations.?

What about mass extinctions? There've been two biggies, end-Permian and end-Cretaceous.?

• At the end of the Permian not only did the trilobites disAppear, but in fact the estimate is that 97% of all marine invertebrate species disAppeared at the end of the Permian. That is an extremely close brush with sterilizing the planet.

• At the end of the Cretaceous the things that disAppeared, ammonites, dinosaurs,?almost everything that lived on land, that was bigger than five kilos, went extinct, and about 70% of the marine invertebrate species went extinct. So this was a big one, but the biggest was the Permian extinction.

What caused that big extinction? Gondwana was breaking up, and Laurasia was also separating from Gondwana--so Pangaea was breaking up. At that time there was large-scale volcanism, and at that time there was a lack of oxygen in the oceans.

• Imagine the entire world's ocean being in a state like Black Sea,?a very thin, oxygenated, clear upper layer; and everything below it basically anoxic: no vertebrates can live in it; it's dominated by bacteria; and it stinks like rotten eggs.

• Some people have suggested that there were extraterrestrial influences at the time. It's been difficult to find a meteorite crater of exactly the right age since?plate tectonics has remodeled the surface of the planet extensively since then, and it's quite possible that there was a big meteorite crater but it got subducted and it's been erased.

• It seems likely that it's the breakup of the continents and large-scale volcanism, but I can't really claim that we really know what caused the extinction. If you go to Siberia, you can find what are called the Siberian traps. These are among the largest flood basalt lava flows on the planet, and they have just the right age; they're at about 251 million years.?

And we know that the?extinction lasted not too long; it was a few 10,000 years. It hAppened both?on land and in the oceans, and the organisms that went out in the oceans were?particularly susceptible to changes in the gas regime. So that would suggest?very high CO2 levels. That's one of the more plausible hypotheses for the end-Permian extinction.

• So one idea is this: there were massive volcanic outbreaks in Siberia. That caused global warming. That global warming then triggered the release of a huge amount of methane that was stored in the ocean.?The methane then gets oxidized to carbon dioxide, and essentially extinctions hAppen by poisoning and asphyxiation.

• We do see a signature in the rocks that indicate that there was an amount of carbon that was oxidized at that point equal to several times the current biomass of the planet. So carbon levels really dropped. At the end of this process the percentage of oxygen in the earth's atmosphere is about 7%.?

The Cretaceous extinction is at just about 65 million years ago; slightly less, 63 and a half, 64 million years ago. And we do know that there was a big meteorite that hit the Yucatan right at the right time. It probably did trigger extinctions. Mechanisms aren't completely clear. It wasn't necessarily the sole cause. That meteorite in the Yucatan could have set off massive volcanism in India, and the reason is this:?

The earth is a spherical lens, and if you throw a big rock into one side of the earth, the energy from the impact radiates out, reflects off the walls of the earth, and comes back together at a single point on the other side. That single point on the other side was focused into western India, at the time that India was moving across the Indian Ocean, before it hit Asia.?And that's where those lava flows are, and those lava flows have exactly the right date.

So that's the end-Cretaceous extinction, and it seems to be linked to the meteorite; and may not only have been caused by the meteorite, there were also volcanic eruptions. I'd now like to do a little bit of local catastrophe--this is on a more frequent timescale--just to convince you that sometimes, on a shorter time period, conditions are quite unusual. Then there are undersea landslides, and these can produce really huge tsunamis.?

So basically the idea of this lecture was to show you that life changed the planet, and mainly it was bacteria that did it; that the planet and the extraterrestrial environment have had occasional major impacts on life. This big picture view, this macroevolutionary view, describes a world that's really qualitatively different from our normal experience.?