Life on Earth
|Life||Life on Earth|
|Life||(kingdoms of life)|
But natheless, whil I have tyme and space,
Er that I ferther in this tale pace,
Me thynketh it acordaunt to resoun
To telle yow al the condicioun
Of ech of hem, so as it semed me,
And whiche they weren, and of what degree,
And eek in what array that they were inne ...
Chaucer, Canterbury Tales: Prologue.
This section is the heart of Palaeos. We have to begin this section somewhere, and so this is also the Beginning of Life. If time permits, we will one day add sections on the definition of "life" and the ways it may have begun. For a recent review, see Robinson 2005). However, that kind of systematic treatment is not exactly what this site is all about (see, infra, the Purpose of Life). So instead, we'll get right to business.
Broadly speaking, we will follow the "Three Domains" model of evolution, at least for organizational purposes. In this model, the earliest and most basic life forms are the Archaea: relatively simple, mostly chemoautotrophic single-celled organisms. Today, these organisms are still often found as "extremophiles," living in harsh, toxic environments using a remarkable diversity of metabolic tricks and adaptations to survive under apparently impossible conditions.
From the Archaea developed two, quite different groups, the Eubacteria and the Eukarya. The Eubacteria are also single-celled organisms, but with complex cell walls, involving peculiar biochemicals which allow us to conclude, with moderate confidence, that they are a natural group. They include the organisms we normally think of as bacteria and also the blue-green algae. The Eukarya include all organisms made up of cells with nuclei. That is, their DNA is walled off in a separate compartment of the cell. Again, this feature is striking enough that we can be reasonably sure that the Eukarya all derived from a single common ancestor, probably within Archaea. Eukaryotes may be single-celled organisms (such as Amoeba or Paramecium), multi-celled organisms (such as oak trees or humans), or at some intermediate stage of organization (like sponges or slime molds).
From the Eukarya evolved three sorts of multi-celled organisms: plants (Chlorobionta), Fungi, and Animals (Metazoa). Animals were originally radially symmetrical, like a simple jellyfish, or lacked any symmetry, like sponges. However, at some point, six or seven hundred million years ago, some animals Bilateria) evolved bilateral symmetry. This sounds trivial, but it was a very significant breakthrough. Most importantly, it allowed segmentation of body sections, so that different segments of the body could specialize for different functions. This required the evolution of an entirely different type of genetic regulation because genes had to operate differently depending on which segment they might be in. This is substantially more complicated than sponge or jellyfish-style development.
From the basic bilaterian plan, two developmental styles evolved. This gets a bit further into developmental biology than we wish to go at the moment. It is sufficient to note that these are the deuterostomes (echinoderms and chordates) and the protostomes (almost everything else). The chordates include the vertebrates, which includes us. Because we are vertebrates, this group has always had a special place, and we refer to all non-vertebrate metazoans as "invertebrates." We will retain this bit of phylogenetic chauvinism in Palaeos for some organizational purposes, until we hear objections from any brachiopods or priapulid worms who might have occasion to offer their comments.
Finally, its important to remember that this site, like life, was not planned. It grew and evolved. It began and, one day, we will become bored and it will die. It may be useful, but it has no overriding mission other than the fun of creating it. So, if you need something thoroughly vetted and organized, try the Shrub of Life. Palaeos is... different.
ATW041029. Public domain. No rights reserved.
Robinson R (2005), Jump-starting a cellular world: Investigating the origin of life, from soup to networks. PLoS Biol 3: e396.
Cladograms, or "trees of life," are appropriate phylogenetic schemes for metazoans multi-celled eukaryotes) but sometimes misleading in other organisms. Nonetheless, we have to have some kind of roadmap, and cladograms at least have the virtue of being testable. The following map is roughly equivalent to a three-color map of the world with just some the major continents sketched in. Although we pretend to start with basal "Life," we know so little of the very first forms that we actually start with LUCA, the Last Universal Common Ancestor. LUCA, whatever it was, is everybody's great-great-great- ... grandparent. Like the Biblical Adam & Eve, LUCA (presumably a single-celled, asexual organism) had at least two offspring, one of which gave rise to the Eubacteria (the usual run of bacteria), and one of which produced the Archaea (peculiar, extremophile bacteria) and the Eukarya (organisms with cells having separate nuclei — like us). This is, of course, a vast oversimplification. However, we have to start somewhere. A very much simplified diagram of the scheme looks like this:
At this level there are many uncertainties. Are the Fungi more closely related to animals (Metazoa) than to plants? Quite likely. Are eukaryotes really more closely related to Archaea, or do they derive from Eubacteria, or from some fusion of the two? Anybody's guess at this point. We look forward to changing our minds at frequent intervals.
ATW040914, modified ATW050513. Public domain. No rights reserved.
Frankly, we're less worried about being wrong than about missing the show completely. The discussions on this site are of quite variable quality, format, accuracy, and style. We're not worried by that, either. As this site has developed, we've learned that the paleo web is dominated by two groups: educators and academic scientists. The educators tend to want everything boiled down to colorful, but tasteless and insubstantial uniformity. The academic scientists tend to be paralyzed by detail. We aim to steer a middle course, avoiding neither the difficult and technical problems, nor the uncertainty inherent in saying anything meaningful about deep time. Truthfully, we scarcely steer at all, but proceed from subject to subject in the manner of a bumper car ride or a destruction derby. We have no overriding mission to educate or provide definitive guidance. Rather, our's is a more self-indulgent attempt to explore the world and to pick up rocks just to see what's under them.
Then again, maybe that is the purpose of life.
ATW030329. Public Domain. No rights reserved.
|(Life on Earth)||(Life)||(Kingdoms of Life)|