The Tree of Life - major groupings and relationships

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The Tree of Life - taken from "San Jose Mercury", September 20, 1999. Article by Glennda Chui. Courtesy of The New York Times.

At the present time, there are diverse opinions among scientists regarding what the major groups of living things are, and how they are related. Even the appropriate metaphor for explaining the relationships between living things is a matter for debate - hand? tree? bush? vine? net? What follows is a brief explanation, culled from a variety of sources, mainly collaborative Web projects produced by biologists around the world - BioMedia Associates (2003), The Tree of Life (Doolittle, 2002), the Expert Center for Taxonomic Identification (Hovenkamp, 2002), and the University of California Museum of Paleontology (Waggoner and Speer, 1994, 1998) - as well as articles by biologists (Doolittle, 2000; Forterre and Philippe, 1999; Margulis, 1989 - 1991) and science reports (Chui, 1999).

Following Aristotle, Linnaeus classified living things into two kingdoms: plants and animals. Members of each kingdom were classified in a nested hierarchy (expanded after Linnaeus' death): phylum (e.g. chordates, or animals with backbones), class (e.g. mammals), order (e.g. primates), family (e.g. hominids, or humans and great apes), genus (e.g. Homo) and species (e.g. sapiens). Scientific acceptance of Darwin's theory of evolution allowed biologists to view this scheme as a tree, with descendant branches diverging from a common ancestral stem. However, in the 19th century, following the work of Darwin and Haeckel, evidence began to accumulate that the two-kingdom scheme was too narrow to express the rich diversity of life.

In 1969, Robert Whittaker proposed his classification of life into five kingdoms: monera (bacteria), protista (now known as protoctista - water-dwelling microbes, such as amoebae), plants, fungi and animals. This scheme is still used by some authors.

The first kingdom, Monera, was supposed to include bacteria, whose cells lack nuclei. Most but not all are single-celled (unicellular) organisms.

Protoctista ("first builders") was meant to include single-celled microbes with nuclei, that were previously labelled protista (protozoa and protophyta), as well as certain multi-celled organisms, such as kelp, that do not belong to the plant, animal or fungi kingdoms. Amoebae, algae, seaweeds, slime moulds, ciliates, diatoms, paramecia and forams belong in this kingdom.

Fungi were placed in a third kingdom, apart from plants, because their life-cycle, feeding habits (they do not photosynthesise) and ecological adaptation are so different.

Plants were defined as multicellular organisms which develop from embryos and whose cells contain chlorophyll, enabling them to make their own food.

Lastly, animals were defined as multicellular organisms that are heterotrophic (feeding on others), diploid (with two sets of chromosomes), and that develop from the fusion/fertilization of an egg and a sperm cell. They follow certain stages during their embryonic development: a morula appears first, then a blastula and finally a gastrula - a hollow sac that forms the embryonic precursor to the digestive tract, by means of which animals ingest nutrients and excrete waste.

The five kingdom scheme co-exists with a scheme dividing life into two main divisions: prokaryotes, or organisms without a cell nucleus, whose DNA floats freely within the cell (bacteria, etc.) and eukaryotes, whose DNA is stored in a nucleus (animals, plants, fungi, and protoctista). However, research in the 1990s showed that prokaryotes are far more diverse than anyone had suspected, and a new three-domain classification has emerged. Prokaryotes are now divided into two domains, Bacteria and Archaea, as different from each other as either is from the third domain (Eukaryotes). Archaea are a newly discovered group of microorganisms that can live in extreme habitats such as thermal vents or hyper-saline water.

The rooting of the tree of life is currently contentious, with a bewildering variety of views (see Appendix). No-one knows which domain arose first, when the domains diverged, or which two are closest to each other. Some researchers have proposed there is no unique "tree of life", but rather a tangled thicket of trees or a net, as Doolittle (2000) describes it. They argue that owing to the prevalence of gene-swapping (lateral gene transfer) between different domains of organisms in the past, different genes in the same organism have different family trees, the same organism can be classified in different ways, depending on which of its genes we examine. They also question the idea that all living things diverged from a common ancestral stock, and suggest that there may be many ancestral roots.

The Tree of Life: a current view. Courtesy of "The Scientist", Volume 16, Issue 18, September 16, 2002. Image: Ned Shaw.

The upshot of this is that at the present time, it would be very unwise to describe any group of living things as "primitive". For the purposes of this thesis, I shall content myself with describing prokaryotes as being structurally (not computationally) the simplest cellular life-forms. After briefly looking at viruses, I shall examine the alleged occurrence of mental processes among bacteria, before going on to discuss protoctista, plants and animals.

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