Early Earth

Here's a geological time scale. Jere Lipps will be talking about events in the evolution of animal life in this portion of the geological column. If we go back to the early history of the Earth, we see we're dealing with billions of years, the origin of the Earth, the earth formed from the accretion of planetesimals probably back about 4.6 billion years ago. The first rocks or the oldest rocks are now at about 3.96 billion years in age from the northwest territories of Canada. From that time, we have a rock record. And indeed, in the so-called Archean there is actually a fossil record of early life. What does that fossil record tell us, if anything, about the origin of life other than it happened?

Let me sort of paint a scenario for what's going on in the early earth to give you an idea. What is the window of opportunity for chemical evolution? Was it hundreds of millions of years or was it tens of millions of years? We do know, based on the genetic code, molecular biology, that life had a last common ancestor. However, that does not mean that life on Earth arose only once. With life on Earth, chemical evolution might have occurred several times. But it's only the last common ancestor that made it. There might have been extinction of that early life. Well, one of the problems is we don't have a rock record at the time of interest, the time of concern. So we have to speculate, unfortunately. But if we knew things, then it doesn't make it a lot of fun. Let's face it, that's really what keeps science ticking. Rich Klein mentioned we don't know about some aspects about the work that he was talking on. And this is really what fuels science, searching for answers, understanding things that we don't know. Science is self-correcting, which is one of its wonderful attributes. We can correct things that were in error in the past, we can reform hypotheses, make them more robust. Science has a wonderful self-correcting aspect.

What I'll be saying today, maybe in a year or two it will not be believed or thought to be as true, or, we may actually find some answers for some of the suggestions that have come up. Let's go back again to these origin of the Earth, 4.6 billion years ago. Planetesimals have accumulated, gravitationally started collapsing, compression, heat, a lot of radioactivity. Soon after, in the earliest history of the Earth after these cold planetesimals collected, the earth basically melted. It was a molten earth and then it cooled off. Geologists as well as planetary scientists, are wondering "when and how soon did the earth actually cool off to the point where there was crust or solid rock existing?" One of the things for the origin of life that is in the chemical evolution experiments that is necessary is liquid water. That is the essential, universal prerequisite for chemical evolution as we understand it based on our Earth forms of life, based on the experiments in the laboratory, liquid water. So, how long did it take for the Earth to cool down, to have rocks on the surface of the earth, solid crusts and have liquid water?

The current estimate is that this may have occurred as early as 4.4 billion years ago. So in about 200 million years, conditions must have existed for a habitable planet. So that gives us quite a bit of time when we consider the oldest fossils are 3.5. So at 4.4 billion years we have a solid crust. However, if you think back on what I said a few minutes ago, the oldest rocks are 3.96 billion years. We're losing over 400 million years of the geological record. Where is that record? What happened to it? Why are rocks, terrestrial rocks, older than 3.96 billion years not found on Earth? This is the critical time to try to find evidence for chemical evolution, to look for some hint preserved in the rock record that may help us out.

So we're going at it from two directions. As you can see, we're starting from the bottom and looking up. The whole question is, here's the origin of life window and we're looking at it from the first record and trying to go back, trying to understand the conditions on the early earth. If we look at the rocks at 3.96 billion years ago, those rocks don't fit what geologists would presume to be the first crust that the Earth would have. The postulate is it would be like a basalt. The rocks suggest that there had been some recycling. It's not really a primitive rock. It's undergone some recycling, some melting, some cooling, erosion of material, remelting. So that implies still a prehistory with the crust, some differentiation with the crust. Interestingly enough, there are some minerals, not the rock from which the mineral came from but individual minerals that were eroded from some preexisting rock, transported, and those mineral grains--like if you look at a sandstone there are individual grains of quartz in that sandstone--that are trapped in a younger rock, that are very old. There are some individual minerals found in a rock. The rock may be about two or three billion years old, that's when the rock formed, the sedimentary rock, but there are little minerals in it that are about 4.1 to 4.2 billion years in age. We don't have the rock but we have the minerals, just like if you look at the sandstone. The individual grain of quartz is older than the sandstone. Those individual minerals are of a chemical composition that also suggests that there was some recycling of these crustal rocks. You wouldn't find these minerals, called a zircon, in a basalt, it's unexpected. It requires a much more mature kind of rock.

So now we can go back 4.1 to 4.2 and it also makes that 4.4 billion years for crust on the Earth more reasonable. We're starting to convert coming from both ends of the spectrum, taking the evidence that we find, projecting that back, taking models for the origin of the Earth and projecting them forward in time and indeed it makes even more reasonable, again, that at 4.4 billion years there was solid rock.

Now, let me throw in paleontology. The oldest fossils are about 3.5 billion years in age. I'll show some pictures of them. And they actually represent what appear to be some complex bacteria. Recently, in rocks 3.8 billion years, isotopes of carbon have been analyzed to suggest that carbon fixation, maybe even photosynthesis was around by 3.8 billion years ago. So now we have evidence of life at about 3.8, some rocks at 3.96, habitability may be as far back as 4.4. What's going on? This is one of the more fascinating aspects with regard to all this and pulls in the interdisciplinary nature of astrobiology and origin of life. There was an interval of time, 3.8 to 3.9 billion years and older, going back to about 4.2 to 4.3 billion, when the Earth and the other inner planets were subjected to a great deal of bombardment by asteroids and comets. It was part of the evolution of the solar system. So you had impactors coming in and slamming into the primitive earth. The moon may have originated from a Mars-sized object striking the Earth, sloughing off a great big chunk of the primitive earth to form the moon. That's one hypothesis. But it was an incredibly energetic time. How frequently did these impactors strike the Earth? Well, that's sort of an interesting question. One per ten million years, one large impact, whatever. It's really, in a sense, I would say, doesn't matter because let's step back and look at chemical evolution. 4.4 billion years ago, habitability, chemical evolution going on, think of what's been achieved in 44 years in the laboratory. Think of the volume of material that's processed in the laboratory. Then think of the primitive earth back 4.4 billion years ago, the natural laboratory.

Probably, chemical evolution experiments were going on all over. Abundant energy resources, heat, even though there's crust it was still probably very warm. Volcanic activity was probably rampant, additional heat. More radioactivity, the energy supplied by the impactors. Indeed, in some of these chemical evolution experiments, I understand if you rap the vessel with a hammer, you can get a few organic compounds forming. So shock waves could do it. Solar radiation could do it. So a variety of energy sources and indeed in the experiments in the laboratories, different model gases are used, different energy sources are used and you get different yields of some of these organic compounds.

So the early earth was just one big chemical evolution experiment. So, maybe a self-replicating cell evolved 4.3 billion years ago: impactor, it's gone. You may have vaporized all the water that was on the early earth and therefore sterilized the early earth. So we had a very, very interesting situation going on where the conditions existed 4.4 billion years ago for chemical evolution to occur and life to originate but we also had a situation in which extraterrestrial bodies were striking the Earth, annihilating that early life. Some people have called it impact frustration in the early appearance of life on Earth.

However, probably the comets were bringing in a lot of water so we're increasing the amount of water that the early earth is getting. After the impact, things could cool off. Comets would also be bringing in organic compounds. If you look at the chemistry of the universe, and look how abundant carbon, hydrogen, nitrogen and oxygen really is in the chemistry of the universe, the universe in a sense has an organic chemistry. So the impactors, although having a great deal of destructive power, they're also bringing in raw materials to the origin of life. And conditions were actually maybe even improving the origin of life, bringing in carbon compounds, bringing in more water, bringing in energy. It's just when you get the really big impactor where all bets are off, where you vaporize the lakes and the oceans that may have existed. But modelers, in particular at NASA, have suggested that if there had been a big ocean back 4.3, 4.4, and chemical evolution and life existed at a vent system, maybe that could survive a large impact. So all may not be lost. But nevertheless, an extremely complex and energetic system existed. Then at about 3.8 to 3.9 billion years ago, this impacting tapered off.

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