Thursday, December 8, 2016

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.
  • Shortly after life appeared, living things evolved and were divided into two large trunks: eubacteria (true bacteria), known since the discovery of microorganisms, several centuries ago, and Archaea, identified only around 1980. Both groups, considered by some biologists to be kingdoms, are classified in the domain prokaryota (unicellular organisms without a nucleus), the most primitive living organisms (viruses may be degenerate incomplete cells adapted to parasitic life).
  • Around 2000 million years ago appeared the third trunk of the tree of life: the domain eukaryota, cells with nuclei and mitochondria. Apparently one archaea engulfed a bacterium, but instead of digesting it, both learned to live together (and to reproduce together), one inside the other, giving rise to a new type of living beings. That first hypothetical eukaryote, this symbiosis of archaea and bacteria, that we cannot find in the fossil record, although we can infer its existence, also has a name: LECA, the acronym of Last Eukaryotic Common Ancestor.
  • Among 1000 and 600 million years ago, some unicellular eukaryota made a second jump and learned to live and to reproduce together, leading to the emergence of a new type of beings: the multicellular. This step happened several times, in several different groups of eukaryotes, so these beings are divided into one one-celled kingdom (protists, eukaryotic cells that remained unicellular) and three multicellular kingdoms: fungi, plants and animals.
In summary, living beings are currently classified as follows:
a) The domain of prokaryotes, unicellular organisms without a nucleus, which in turn are divided into two kingdoms: bacteria and archaea.
b) The domain of eukaryotes, unicellular with nucleus and multicellular, which in turn are divided into a unicellular kingdom (protists) and three multicellular kingdoms: fungi, plants and animals.
The former is the systematic point of view, related to classification. From the point of view of the degree of complexity, we have four levels of progressive organization:
1. Prokaryote level: unicellular organisms without a nucleus.
2. Unicellular eukaryote level: unicellular organism with a nucleus, which came into existence when one archaea and one bacterium learned to live together.
3. Multicellular eukaryote level: individuals made of many unicellular cells that make life together.
4. Human level: a single species of multicellular eukaryotes has crossed a critical point. Man is the only living being able to evolve in two ways at once: biological evolution, based on our genome (this is common to all living beings) and cultural evolution, which only happens in man.

Our closest relatives (chimpanzees) show signs of approaching the critical point, but have not yet gone through it, in the same way that water at 99°C gives off vapors, but is still liquid. The difference between the chimpanzee and man can be summed up in a phrase in one of my novels, Los moradores de la noche (Dwellers of the night):
Man studies chimpanzees. Chimpanzees do not study man.

A few related links in this blog:

Manuel Alfonseca

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