We have living organisms, although no one is really sure how that happened. But, before I get into that discussion, I want to introduce a couple of things that are critically important.
First, plate tectonics, also known as continental drift. We're at a point where we have landmasses and oceans on a planet that is still geologically very active. Those landmasses, which we will call "continents," move around. They still do, because the planet is still active. Earth's crust is divided into plates that are subject to the influences of convection currents in the planet's mantle, that layer of molten rock between the crust and the core. Where the currents rise, the crust splits -- what we call "rifts." Where the currents sink, so do the edges of the plates -- subduction: in other words, the edge of one plate moves under the edge of the adjoining plate, leading to lots of earthquakes, volcanoes and things like the Himalayes, the Andes, the Rockies -- pretty much every mountain range that ever existed. (It is possible for mountain ranges to be created other ways -- such as when the crust moves over a "hot spot" -- the Hawaiian Islands and the Galapagos are good examples: they're really huge mountains -- sea mounts -- created as the crust moved over a hot spot on the ocean floor.)
It's worth remembering that the plates are not static. In addition to moving around, they split and combine over time. Here's a nice animation of the movement of landmasses over time, created by a user named Algol at YouTube:
You'll notice that there seems to be a lot more land at the end of the video than there was at the beginning. In part this is due to fluctuations in sea level, but of more import, I think, is the fact that more land was being created by volcanic eruptions: a lot of that molten rock from the interior is now on the surface. However, in spite of all this activity, we can still find rocks that date nearly to the creation of the planet.
Continental drift is important because continents moving around have a huge influence on climate. One of the Great Extinctions resulted from a large continental mass moving over the South Pole, which lowered global temperatures and sea levels as the water in the oceans became locked into snow and glaciers. And climate is a key element of the environment to which organisms must adapt -- climate, along with geography, determines whether a given area is desert, rain forest, savannah, or whatever. If I can, I'll include maps showing the contemporaneous distribution of continents as we go along.
The second thing is evolution. In spite of what you've been told by your creationist uncle, evolution does not have a thing to do with the origin of life. It kicks in once life has already happened. One thing I'd like to point out about how it works: all too often, we hear people say (even people who should know better) say that such and such an organism evolved to fit such and such an environment. Not really. In fact, that's just about the opposite of what happens.
Evolution happens in populations, and thanks to sex, populations have a fair amount of genetic variability -- some individuals will have traits that others lack and vice-versa. (We'll get to sex later -- it hasn't happened yet.) One or a combination of these traits may allow an individual to take advantage of a new environment. The descendants of that individual will, for the most part, possess that trait and are more successful at exploiting that environment than their fellows who lack that trait. This is what Darwin meant by "natural selection": the environment exerts selection pressure that favors certain traits over others. This will eventually lead to the appearance of new species and, early on, to new orders of life. Remember, we all started out as a single-celled organism.
And in that regard, I will probably be including from time to time what are called "cladograms" -- diagrams that show the descent of various groups from common ancestors, based on what we know about their relationships from genetics, morphology, biochemistry, and the fossil record. The image to the left is a sample of what a cladogram looks like -- I think this one is mostly concerned with fishes.
I should also mention here geologic eras. Earth's history is divided into sections (as you might well imagine -- 4.5 billion years can be pretty much unmanageable without some markers along the way). The largest are eons, which may be composed of several eras; subdivisions of eras are named periods, and further subdivisions, the smallest segments, are called epochs. The Hadean Era, which we've already encountered, is usually considered part of the Precambrian, which is where we are. (I'm probably not going to deal with anything larger than periods here.):
And that's enough for now. I'll get into the thorny question of how all those organic molecules got themselves organized into an actual organism next week.
It's worth remembering that the plates are not static. In addition to moving around, they split and combine over time. Here's a nice animation of the movement of landmasses over time, created by a user named Algol at YouTube:
You'll notice that there seems to be a lot more land at the end of the video than there was at the beginning. In part this is due to fluctuations in sea level, but of more import, I think, is the fact that more land was being created by volcanic eruptions: a lot of that molten rock from the interior is now on the surface. However, in spite of all this activity, we can still find rocks that date nearly to the creation of the planet.
Continental drift is important because continents moving around have a huge influence on climate. One of the Great Extinctions resulted from a large continental mass moving over the South Pole, which lowered global temperatures and sea levels as the water in the oceans became locked into snow and glaciers. And climate is a key element of the environment to which organisms must adapt -- climate, along with geography, determines whether a given area is desert, rain forest, savannah, or whatever. If I can, I'll include maps showing the contemporaneous distribution of continents as we go along.
The second thing is evolution. In spite of what you've been told by your creationist uncle, evolution does not have a thing to do with the origin of life. It kicks in once life has already happened. One thing I'd like to point out about how it works: all too often, we hear people say (even people who should know better) say that such and such an organism evolved to fit such and such an environment. Not really. In fact, that's just about the opposite of what happens.
Evolution happens in populations, and thanks to sex, populations have a fair amount of genetic variability -- some individuals will have traits that others lack and vice-versa. (We'll get to sex later -- it hasn't happened yet.) One or a combination of these traits may allow an individual to take advantage of a new environment. The descendants of that individual will, for the most part, possess that trait and are more successful at exploiting that environment than their fellows who lack that trait. This is what Darwin meant by "natural selection": the environment exerts selection pressure that favors certain traits over others. This will eventually lead to the appearance of new species and, early on, to new orders of life. Remember, we all started out as a single-celled organism.
And in that regard, I will probably be including from time to time what are called "cladograms" -- diagrams that show the descent of various groups from common ancestors, based on what we know about their relationships from genetics, morphology, biochemistry, and the fossil record. The image to the left is a sample of what a cladogram looks like -- I think this one is mostly concerned with fishes.
I should also mention here geologic eras. Earth's history is divided into sections (as you might well imagine -- 4.5 billion years can be pretty much unmanageable without some markers along the way). The largest are eons, which may be composed of several eras; subdivisions of eras are named periods, and further subdivisions, the smallest segments, are called epochs. The Hadean Era, which we've already encountered, is usually considered part of the Precambrian, which is where we are. (I'm probably not going to deal with anything larger than periods here.):
And that's enough for now. I'll get into the thorny question of how all those organic molecules got themselves organized into an actual organism next week.
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