Life

From Palaeos

        H.Ar.Pz.Pz.Mz.Cz.R.

What is life? How did it come about? Why is it still around? Does it change? These and other questions are discussed, and hopefully answered in this section of Palaeos. Life is one of the great enigmas of science. Everything else in the universe resolutely obeys the second law of thermodynamics: Thou shalt become less complicated over time. This is commonly known as entropy. Life flamboyantly flouts this regulation, getting successively more complicated with every chance it gets. Life also seems to take every oppurtunity to make more of itself. Called self-replication, this ability, though not the defining characteristic of life, is frequently cited as an important facet of life's identity. Another such facet is life's ability to adapt. Not only do living things make more of themselves, they get better at doing so, and surviving to do so, with every new generation. This process of contiual change is called evolution. These three qualities are what will be looked for in any search for extraterrestrial life: violation of entropy, self-replication, and evolution.

Life in the Universe

Life on Earth

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. 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.

The Evolution of Life on Earth

Broadly speaking, this site follows the "Three Domains" interpretation of the evolutionary tree of life. This divides living organisms into three primary types, two of which were formerly referred to simply as bacteria.

Archaea (also called Archaeabacteria) are currently thought be closer to eukarya (i.e. us) than to bacteria (Eubacteria), though some worker still believe they are the most basal of the three domians. They are anaerobic and chemautotrophic; fancy words that mean they don't require oxygen for metabolism. All are single-celled organisms, called prokaryotes, which means that their DNA is not separate from the rest of the cell. Originally the predominant form of life during the Archean eon, the oxidization of the reducing atmosphere and evolution of new more efficient aerobes mean that archaeabacteria have retreated to harsh, toxic environments (the technical term is "extremophiles") using a remarkable diversity of metabolic tricks and adaptations to survive under apparently impossible conditions.

The Eubacteria are, like the Archaea, also very simple single-celled prokaryote organisms, but with complex cell walls. These cell walls involve peculiar biochemicals that are the same in all Eubacteria, which seem to indicate 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 (genetic material) is in a separate compartment to the rest of the cell. It is thought that like the Eubacteria they can all be derived from a single common ancestor, in this case probably within Archaea.

Although ancestral Eukarya presumably shared the prokaryote condition of their Archaean and Eubacterial brethren, all living Eukarya have a distinct nucleus. Lynn Marguilis has popularised the thesis that the eukaryote cell is actually the symbiotic merger of a number of originally distinct prokaryote organisms.

The term Last Universal Common Ancestor (LUCA) is used to refer to the ancestor shared by all three groups. Despite their name, the Archaea may not be the original ancestors. Research indicates that the Archaea and the Eukarya may actually be more closely related to each other than either is to the Eubacteria. Alternatively, the three domains may have arisen equally out of an original assemblage of basal prokaryotes, in which genetic material was freely exchanged, making precise phylogentic analysis impossible, as the problem of lateral gene transfer makes the base of the "Tree of Life" reticulate (compared to dichotomous).

Eukaryotes include single-celled organisms (such as Amoeba or Paramecium), which were originally included in the kingdom Protista (a term no longer used because it is a grade rather than a natural group), multi-celled organisms (such as oak trees or humans), or at some intermediate stage of organisation (like sponges or slime molds).

Multi-celled organisms have evolved at least five times with Eukarya: green plants (Chlorobionta), Fungi, Animals (Metazoa), red algae (Rhodophyta) and brown algae (Phaeophyta).

Animals were originally radially symmetrical, like a simple jellyfish, or lacked any symmetry, like sponges. In non-technical terms this means that the first animals lacked any front or rear end, no head (and hence no brain) and no tail. They could move equally in any direction or none. However, at some point, six or seven hundred million years ago, some animals evolved bilateral symmetry. That is, the front of the animal became different in shape and form to the rear. 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 specialise for different functions. The evolution of an complex genetic regulation, called "Hoxbox" genes is far more extensive in bilateral animals than radially symmetric ones. Genes now 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 in the very early embryo evolved. In one, the protostomes, the mouth develops before the anus. In the other, the Deuterostomia, the mouth develops second.

The deuterostomes include phyla such as the echinoderms (starfish, sea urchins, etc) and the chordates. Despite their great difference in form and organisation, these two groups are actually related. The chordates in turn include the vertebrates, which includes us. Because we are vertebrates, this group tends to be given more attention in works on the phylogeny of life, with all non-vertebrate metazoans classified as "invertebrates." This bit of phylogenetic chauvinism will probably be retained in the Palaeos Wiki, at least for now.

Mapping the Tree of Life

Cladograms, or "trees of life," are useful phylogenetic diagrams for multi-celled eukaryotes, but sometimes misleading in other organisms. But we need to have to have some kind of roadmap, and cladograms at least have the virtue of being testable. The following is an extremely simple cladogram (or more properly a dendrogram or phylogenetic tree); elsewhere in this site will be more detailed ones. The diagram is read from top left to bottom right. Probable paraphyletic taxa (which are evolutionary gardes rather than groups united by a single unique common ancestor) are indicated by double horizontal ( and ) lines to indicate that there are actually a number of specific clades in these cases.

LUCA ►
Eubacteria ►
Archaea ►
Eukarya ►
Chlorobionta ► (green plants)
Fungi ►
Metazoa ► (animals)
Porifera ►
Radiata ►
Bilateralia ►
Protostomia ►
Mollusca ►
Arthropoda ►
Deuterostomia ►
Echinodermata ►
Vertebrata ►

Note that this diagram should not be taken as a dogmatic fact. There are still many points of uncertainty. 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. And the more detailed the cladograms, the more uncertainties and alternative explanations, and rival theories

The Purpose of Life

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. (Or, of course, inside.)

Then again, maybe that is the purpose of life.

Hadean  Archean  Proterozoic  Phanerozoic 
     LUCA
       |--Eubacteria ---------------------
       `--Archaea  -----------------------
            `---- Eukarya ----------------
                     |--Plantae  ---------
                     `-----+--Fungi ------
                           `--Metazoa ----

Palaeos com - Kingdoms of Life

Credits: Palaeos com page ATW041029, ATW030329, and ATW040914, modified ATW050513 (Public domain). Modified MAK060920 MAK060928,