The three trunks of the tree of life

The tree of life

As time goes by and more and more genomes of living beings of very different types are sequenced, we are learning a lot about the tree of life. This is a summary of what we know:

• From the fact that all current living things use the same genetic code (with very minor variations) it follows that all the living beings we know, current or extinct (including viruses), descend from a single ancestor, unknown, of course, because there is no trace of in the fossil record, and if we found it, we would not recognize it. This hypothetical common ancestor has received the curious name LUCA, the acronym of Last Universal Common Ancestor. The first living creature should be placed at the very origin of the tree of life (in the root). Many biologist also think that this common ancestor appeared over 3000 million years ago, near the hydrothermal vents found on the mid-ocean ridges that separate the plates of the earth’s crust, where the magma in the mantle tends to rise to the surface.

Sex and species, two related concepts

Taxonomic categories

The species is the basic taxonomic category used by biologists to classify living things. The other categories (genus, family, order, class and phylum) are considered artificial and arbitrary. On the other hand, we tend to regard the species as natural, obvious, similar to a concept when the represented objects are living beings. But we will not enter here into the famous problem of universals, nor wonder on whether concepts (and species) really exist or are mere constructs of the human mind.

The classic definition of a species is: a set of living beings that share common characteristics and can interbreed, giving rise to fertile offspring. Notice that the use of the word interbreed implies that the living things in question use sexual reproduction. This leads us to ask whether the concept of a species should be restricted to living beings with this type of reproduction, or it can be extended to those that reproduce otherwise, such as prokaryotes and some eukaryotes. This question can be answered in several ways:

The origin of sexual reproduction

Cromosomas X e Y

Sexual reproduction is the most common form of reproduction among eukaryotes, including multi-cellular living beings. After billions of years of asexual reproduction among prokaryotes, who share genetic information by exchanging plasmids (small DNA fragments), a new type of reproduction suddenly appeared. As it was successful, we must assume that it must have provided some advantage over the alternative procedure.

Sexual reproduction can be defined as the alternation between two life cycles for the same type of organism:

• Haploid cycle: each cell has a single copy of every chromosome.
• Diploid cycle: each cell has two copies of every chromosome.

In eukaryotes, the haploid cycle is always unicellular; the diploid cycle may be unicellular (in unicellular eukaryotes) or multi-cellular (in multi-cellular eukaryotes). The individuals in the haploid cycle are called gametes.

How was this alternation established? It could have been caused by an alternation between two different environments. Haploid and diploid cells do not have the same properties. For example, diploid cells are more voluminous, having many duplicate organelles, so that the ratio of surface to volume is usually larger than in haploid cells (about 1.25 times). As the absorption of nutrients by the cells depends on their surface, haploids tend to grow faster than diploids.