Before we dive headlong into the Cambrian Era and all the excitement that was part of it, a few words about some basics of evolution.
It's important to remember the evolution -- that is, change over time -- operates in populations. For example, you have a species adapted to a particular environment -- say, an area bordering on a wetland. A group of individuals of that species starts to move into a slightly different environment -- let's say slightly farther from the wetland, up in the hills, where it's drier. Because sexual reproduction gives genetic variability in a population, some members of that group are able to take better advantage of the new environment -- they don't need to drink as often, or they are better able to obtain moisture from their food. Those individuals then are stronger and healthier and will produce more offspring, which inherit those favorable characteristics. After enough time has passed, this new population becomes a new species -- that is, in the classical meaning of the term, they no longer interbreed freely with the parent population. (At some point, probably soon, I will discuss the vagaries of taxonomy and how nature tends not to pay attention to our ideas about how it should operate.) Thus, through the operation of genetic variability in a particular environment, we have a new genotype: the population has evolved.
The other major factor here is geography, which is a basic and essential component of the environment. You may remember that long, long ago, when this all started, there wasn't very much land on earth. What there was was the result of vulcanism -- lots of volcanoes and such. Over time, as the rains came and rivers formed, sediments began to be added to the mix -- sandstone and the like -- and with the advent of skeletons in single-celled organisms, we have limestone. (Pinning down the earliest limestone is next to impossible, simply because it erodes so easily. This article discusses some of the problems in finding early specimens, and also gives a hint that limestone may very well predate the most common estimate of origins in the early Cambrian -- early fossils have been found in rocks that most likely predate the Cambrian, that is, they are older than about 535 million years.)
It's also worth remembering that the land wasn't uniform -- there were mountains, there were valleys, there were lots of variations in topography. This held true as much for the underwater portions of the earth as for dry land. Thus, even at this early date, we have a series of different environments largely determined by the shape of the land. All this land-building is important because most of what we're going to be discussing happened on land, although the great migration from sea to land isn't going to happen for a couple hundred million years.
So, by the beginning of the Cambrian, after a couple billion years of land-building, we have something like this:
This more or less sets the stage for the Cambrian Explosion, that period starting about 550 million years ago when all the present-day animal groups first made their appearance. Here's a taste of what we're in for.
It's important to remember the evolution -- that is, change over time -- operates in populations. For example, you have a species adapted to a particular environment -- say, an area bordering on a wetland. A group of individuals of that species starts to move into a slightly different environment -- let's say slightly farther from the wetland, up in the hills, where it's drier. Because sexual reproduction gives genetic variability in a population, some members of that group are able to take better advantage of the new environment -- they don't need to drink as often, or they are better able to obtain moisture from their food. Those individuals then are stronger and healthier and will produce more offspring, which inherit those favorable characteristics. After enough time has passed, this new population becomes a new species -- that is, in the classical meaning of the term, they no longer interbreed freely with the parent population. (At some point, probably soon, I will discuss the vagaries of taxonomy and how nature tends not to pay attention to our ideas about how it should operate.) Thus, through the operation of genetic variability in a particular environment, we have a new genotype: the population has evolved.
It's also worth remembering that the land wasn't uniform -- there were mountains, there were valleys, there were lots of variations in topography. This held true as much for the underwater portions of the earth as for dry land. Thus, even at this early date, we have a series of different environments largely determined by the shape of the land. All this land-building is important because most of what we're going to be discussing happened on land, although the great migration from sea to land isn't going to happen for a couple hundred million years.
So, by the beginning of the Cambrian, after a couple billion years of land-building, we have something like this:
This more or less sets the stage for the Cambrian Explosion, that period starting about 550 million years ago when all the present-day animal groups first made their appearance. Here's a taste of what we're in for.
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