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| 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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