Biology: History of Life: Heterotroph Hypothesis

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You notice that the title of this lecture has some heterotroph hypothesis. Before we do a whole bunch of facts and pull some stuff together, I really need you to understand the difference between a theory and a hypothesis. In many ways this becomes the essence of understanding science of any kind. Let me start out with a question. How many of you have ever seen an atom? Okay, none of you raised your hands. That's good, because no one has ever seen an atom, no one. I'm going to ask you a second question. How many of you believe in atoms? Very good, very good, it's nice to know you all believe in atoms. Now, do you realize that you believe in something no one's ever seen? Why is that?
Well, it's because you've been raised in the scientific age. You see, we have something called the atomic theory. It's all theoretical. We believe in atoms because things behave like they're made out of atoms. We believe in electrons because we say, "Ah, we can move them through wires and light bulbs and stuff like that." So, we have evidence, you see, we have evidence, overwhelming evidence that atoms exist. So, in science the idea of theory is kind of different than vernacular. Vernacular, common speech says, "Well, it's kind of theoretical." Theoretical, at least, in common speech and nonscientific terms, means it may be true or it may not be true. But at science we draw the line. There is a bit of confusion about that.
When something is theory in science it's as close to--I'm going to use the word truth. Bad word to use in science, because there are no absolute truths. But it's as close to fact as science can bring it. To be called a theory there has to be overwhelming evidence for something to exist. Like I said, there are no absolutes in science. Tomorrow one of you may go out and proof that there are no atoms and as soon as you do that, I'm with you. But for now, I accept the principle of the atom. We have been talking a lot about the theory of evolution. I was recently talking to someone and they said, "You know, I really have the theory or evolution, because it talks about the fact that life came from a hot thin soup and the primordial ooze and stuff like that."
I stopped right there and I said, "Time out." We really have not much evidence as to how life evolved. Thus, it is still very much at the hypothetical question of the heterotroph hypothesis. We have, as you're going to see, a very big body of evidence growing as to how life evolved. But no scientist will say to you, "This is how life started." We can tell you how we think it started. We have evidence that it may have started this way, but it is still very hypothetical. I think it's crucial for you guys to really internalize the difference between a theory and a hypothesis.
Okay, off the podium and back to some very cool facts. So, what is this hypothesis? It is a very intriguing and exciting question. Mountains of Ph.D. theses have been written about how biotic life formed. There's been some great experiments that have shown that too. Well, let's talk about life and let's talk about the age of the planet. As best as we can ascertain from radioactive isotopes, the oldest rocks we have found on the planet are about 4.5 billion years. That's how old our planet is, as far as we know. How it got here, well that's a job for you physics folks to talk about, but the bottom lie is the oldest rock we can find 4.5 billion years.
When all of a sudden in the fossil record do we start finding life forms? Well, I've got to tell you about a very cool fossil called a stromatolite. A stromatolite, and by the way these things are still forming today. A stromatolite is these kind of dome shaped objects that we find. There's some particularly good formations of stromatolite in Australia. If you look at a stromatolite really up close, what you find is that it's layered. Very, very layered in sedimentary bands. If you do a microscopic analysis of that stromatolite, you find out that indeed the stromatolite is fossils of bacteria. Bacteria, a group that we'll talk about later called cyanobacteria, one celled microorganisms.
These one-celled micro organisms are indeed 3.5 to possibly 4 billion years old. Think of that. Let's just take the small one, 3.5 billion years old. You know what that means? That means that life developed pretty rapidly on this planet. When we go to rock that's 4.5 billion years old, we find no life forms at all. When we go to rock that's 3.5 billion years old, we find one celled life organism. We don't find fish, we don't find horse, we don't find cows. We find one celled life forms. Within hundreds of millions of years this planet went from a planet that supported no life, according to the fossil record, to a planet that had one-celled creatures. Incredible stuff.
Well, let's talk a little bit about these one celled creatures. These one celled creatures, and we're going to do a lot on classification later on in the course, but I have to lay two words on you. Prokaryote and Eukaryote. Now, these are descriptive terms that talk about cells, cells on the planet. We're going to see later that Prokaryotes is a generic descriptive term that describes what now are kind of being broken down into a group called bacteria, which you're all familiar with and a group that use to be called bacteria but are now separated from the bacteria. They're called Archaea. Don't worry too much about, why just separate them, we're getting there. That's kind of cool stuff. Don't fast forward, we'll get there later.
The Prokaryotes are ancient cells. In fact, the whole word Prokaryote means that these cells lack a nuclear membrane. Now, I hate to define something in terms of what it lacks because they have an awful lot of stuff, but just in terms of fossil record and identification we have to say it that way. You Eukaryotic cells, like yours, have a membrane around them, have a nucleus. It turns out that Prokaryotic cells, no nucleus and no nuclear membrane, that Prokaryotic cells are the ones that have sedimented into the stromatolites. So, what we have to do in the rest our lectures is this. We have to account for that hundred million years. We have to account for where did those Prokaryotes come from. How did they evolve? What kind of chemical processes would have occurred and what happened after they occurred? We're moving on to the potential question of the origin of life. Where did it come from? How did it start and where do we go from there?
Evolution
The Origin of Life
History of Life: The Heterotroph Hypothesis: An Overview Page [1 of 2]