At long last. It's been a bit weird around here, but I'm getting back on track.
The story so far: Over the course of about three billion years or so, after earth cooled off a bit after its formation, we've seen living organisms develop from organic molecules in the environment. We're not sure how these molecules were organized (or organized themselves) into actual critters, but they did, first simple single-celled organisms, prokaryotes, and eventually eukaryotes, which are the forerunners of pretty much everything. Those first organisms are actually an assumption: in rocks dating to 3.8 to 3.5 billion years ago, there are traces of carbon in patterns similar -- very similar -- to those left by bacteria today.
The prokaryotes -- Archaea and Bacteria -- are still with us, but they haven't developed all that much from where they were 600 million years ago. The big difference, and it is a big one, is that the eukaryotes have a cell nucleus, which contains the cell's DNA, and various organelles, some with their own DNA, that control various processes within the cell. A couple of important points: we have fossil records of organisms with exoskeletons from about 550 million years ago (hereinafter known as "mya"). To state the obvious, this is a strong indicator that we already have fairly complex organisms that may be divided into "predators" and "prey." (The major reason to develop an exoskeleton is that something wants to eat you.)
And somewhere along the line, they discovered, or invented, sex, which is critically important: before this, reproduction was asexual, producing offspring that were exact duplicates of their parent (usually -- sometimes mutations would happen that would introduce some variation, but this was a rare and chancy occurrence). The advantage of sex is that it automatically introduces a range of characteristics into the offspring, providing them with a much greater ability to adapt to changing conditions -- and, as we've seen, conditions were pretty much constantly changing. (A side note: I can't find any contemporary references, but I seem to remember from reading long ago that in the early stages, sexual reproduction was pretty much a free-for-all: there were no species yet. At some point regulatory genes developed that halted the exchange of DNA between unlike organisms: we now have species.)
Now, this may seem like a lot of assumptions with no hard evidence, but we know that the earth started out with organic compounds and somewhere along the line living creatures appeared. We're talking, from the very beginnings to the opening of the Cambrian, roughly three billion years. Remember that evolution operates through generations, and that even without sexual reproduction, there are genetic variations caused by mutations. And remember that the length of a generation for a single-celled organism is about fifteen minutes. That's a lot of generations, and potentially a lot of mutations, and when you factor in sexual reproduction, which introduces even more variability into the genome, pretty much anything is possible. And so we arrive at the Cambrian Period.
Let's take a look at what the world looked like at the point:
As you can see, most of the land was concentrated in the Southern Hemisphere, and there was a lot of ocean. Since the land was pretty much uninhabitable, that was a good thing.
At the beginning of this period, we have some multicellular organisms -- simple sponges or jellyfish-like animals, most of which were stationary, although some did move around. The majority of organisms were single-celled and formed a mat on the ocean floor, feeding directly on the minerals in the rocks. Some of the multicellular animals fed on them.
There's some debate as to what caused what happened next. The prevailing theory has been that the increase in oxygen levels (due, as you'll no doubt remember, to the emergence of cyanobacteria and their relatives, which introduced photosynthesis into the mix, releasing free oxygen as a waste product) made multicellular organisms possible. Some researchers are maintaining that the oxygen levels not only varied over time, but were not great enough to permit the existence of larger multicellular organisms. (Remember that we're dealing with organisms that mostly don't have respiratory systems yet: they're getting their oxygen, as well as their daily sustenance, directly from the environment.) I think it's apropos at this point to introduce the concept of threshold events: things may go along close to the status quo while conditions are changing, and then you reach a tipping point that involves sudden, dramatic changes. We'll see this again and again in the course of life on earth.
What makes this event, or series of events, so important is that almost all the major groups of organisms -- phyla, to be technical -- make their first appearance. And these are the phyla that still exist today (with a few exceptions that didn't make it this far). To give you an idea of what it might have looked like:
Looks pretty alien, doesn't it? But just about all of those kinds of organisms are still around -- or their descendants are.
The Cambrian Explosion necessarily runs into the Ordovician period: diversity increased, some organisms vanished, and some began to colonize the land. By this point there is enough free oxygen in the atmosphere to form an ozone layer, filtering out the worst of the sun's ultraviolet, which had before this kept living organisms confined to the oceans. So, some brave bacteria, fungi, etc. essayed that new environment, sticking close to the water, and perhaps living in the shallows. The end result of this is that as they digested the mineral-rich sediments, and then died and decayed, they created soil that was capable of supporting life. (There is evidence that some animals were exploring the land well before this -- about 530 mya; there is no evidence, however, that they hung around for any length of time.) The first plants -- and probably a few arthropods -- were thus able to survive, although they, too, stuck close to the water: they hadn't yet evolved the means to survive without it.
So we're ready for the invasion of the land.
The story so far: Over the course of about three billion years or so, after earth cooled off a bit after its formation, we've seen living organisms develop from organic molecules in the environment. We're not sure how these molecules were organized (or organized themselves) into actual critters, but they did, first simple single-celled organisms, prokaryotes, and eventually eukaryotes, which are the forerunners of pretty much everything. Those first organisms are actually an assumption: in rocks dating to 3.8 to 3.5 billion years ago, there are traces of carbon in patterns similar -- very similar -- to those left by bacteria today.
The prokaryotes -- Archaea and Bacteria -- are still with us, but they haven't developed all that much from where they were 600 million years ago. The big difference, and it is a big one, is that the eukaryotes have a cell nucleus, which contains the cell's DNA, and various organelles, some with their own DNA, that control various processes within the cell. A couple of important points: we have fossil records of organisms with exoskeletons from about 550 million years ago (hereinafter known as "mya"). To state the obvious, this is a strong indicator that we already have fairly complex organisms that may be divided into "predators" and "prey." (The major reason to develop an exoskeleton is that something wants to eat you.)
And somewhere along the line, they discovered, or invented, sex, which is critically important: before this, reproduction was asexual, producing offspring that were exact duplicates of their parent (usually -- sometimes mutations would happen that would introduce some variation, but this was a rare and chancy occurrence). The advantage of sex is that it automatically introduces a range of characteristics into the offspring, providing them with a much greater ability to adapt to changing conditions -- and, as we've seen, conditions were pretty much constantly changing. (A side note: I can't find any contemporary references, but I seem to remember from reading long ago that in the early stages, sexual reproduction was pretty much a free-for-all: there were no species yet. At some point regulatory genes developed that halted the exchange of DNA between unlike organisms: we now have species.)
Now, this may seem like a lot of assumptions with no hard evidence, but we know that the earth started out with organic compounds and somewhere along the line living creatures appeared. We're talking, from the very beginnings to the opening of the Cambrian, roughly three billion years. Remember that evolution operates through generations, and that even without sexual reproduction, there are genetic variations caused by mutations. And remember that the length of a generation for a single-celled organism is about fifteen minutes. That's a lot of generations, and potentially a lot of mutations, and when you factor in sexual reproduction, which introduces even more variability into the genome, pretty much anything is possible. And so we arrive at the Cambrian Period.
Let's take a look at what the world looked like at the point:
 |
| Early Cambrian continnents. Image: New World Encyclopedia |
As you can see, most of the land was concentrated in the Southern Hemisphere, and there was a lot of ocean. Since the land was pretty much uninhabitable, that was a good thing.
There's some debate as to what caused what happened next. The prevailing theory has been that the increase in oxygen levels (due, as you'll no doubt remember, to the emergence of cyanobacteria and their relatives, which introduced photosynthesis into the mix, releasing free oxygen as a waste product) made multicellular organisms possible. Some researchers are maintaining that the oxygen levels not only varied over time, but were not great enough to permit the existence of larger multicellular organisms. (Remember that we're dealing with organisms that mostly don't have respiratory systems yet: they're getting their oxygen, as well as their daily sustenance, directly from the environment.) I think it's apropos at this point to introduce the concept of threshold events: things may go along close to the status quo while conditions are changing, and then you reach a tipping point that involves sudden, dramatic changes. We'll see this again and again in the course of life on earth.
What makes this event, or series of events, so important is that almost all the major groups of organisms -- phyla, to be technical -- make their first appearance. And these are the phyla that still exist today (with a few exceptions that didn't make it this far). To give you an idea of what it might have looked like:
Looks pretty alien, doesn't it? But just about all of those kinds of organisms are still around -- or their descendants are.
The Cambrian Explosion necessarily runs into the Ordovician period: diversity increased, some organisms vanished, and some began to colonize the land. By this point there is enough free oxygen in the atmosphere to form an ozone layer, filtering out the worst of the sun's ultraviolet, which had before this kept living organisms confined to the oceans. So, some brave bacteria, fungi, etc. essayed that new environment, sticking close to the water, and perhaps living in the shallows. The end result of this is that as they digested the mineral-rich sediments, and then died and decayed, they created soil that was capable of supporting life. (There is evidence that some animals were exploring the land well before this -- about 530 mya; there is no evidence, however, that they hung around for any length of time.) The first plants -- and probably a few arthropods -- were thus able to survive, although they, too, stuck close to the water: they hadn't yet evolved the means to survive without it.
So we're ready for the invasion of the land.
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