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| John Maynard Smith |
As I have said several times in this blog, the theory of evolution is now well established. However, it is far from explaining everything. Many mysteries still remain. I listed some of them in a previous post. A book by J. Maynard Smith and E. Szathmáry, The Major Transitions in Evolution (Oxford University Press, 1995), describes them in more detail.
One of these problems refers to the changes of level that have taken place in the history of life, which I made the central idea of my book The Fifth Level of Evolution. As its title implies, during evolution, things have not happened in an orderly or stable manner. At various points, there were changes of state (similar to those in physics) where evolution passed a critical point that made it possible to reach higher levels and opened up huge new fields in the configuration space. These points are the following:
1.
The origin of proteins (chains of amino acids) and nucleic
acids (chains of
nucleotides). The latter are capable of reproducing with variation, so
according to Muller's definition (but not Maynard Smith's), they should be
considered living beings. This debate aside, their appearance marked the
beginning of the evolution of life (or toward life) on Earth. This is what I
call the
first level of evolution.
2.
The origin of prokaryotic living cells. For many, this is the true origin of life. For
me, it is the transition to the
second level of evolution.
3.
The origin of eukaryotic cells as the final result of the symbiosis of an
archaeal prokaryotic cell and a bacterium, in which the two began to live
together and the latter transformed into the mitochondria of the eukaryotic
cell. This is what I call the
third level of evolution.
4.
The origin of multicellular beings, composed of a more or less large number of
eukaryotic cells that live together and renounce their own reproduction
(sometimes even their lives) for the sake of the higher-order being of which
they are a part. I call this stage the fourth level of evolution.
5.
The origin of societies of living beings, in which a group of individuals from the fourth
level act together to form a higher-order individual. This fifth level of
evolution is in its infancy, with just a few entities, such as coral reefs and
siphonophores among coelenterates; insect societies (termites, ants, bees, and
wasps); the naked mole-rat among mammals; and, above all, human society.
In several publications, I have said that evolution is
not, as some
biologists claim, an undirected process in which all species of living beings have equal
importance. To prove this, I have presented a figure like this, which shows
that the amount of information available to a species has increased over time,
from the origin of life to the present. In the beginning, all information was
genetic; starting with animals, neural information was added; with the arrival
of humans, a huge qualitative leap occurred, with the addition of cultural
information to the previous two. The origin of humans was, therefore, a new
critical point in evolution.
A team of four Spanish researchers has published an article where
they make an important step toward solving the problem of the leap from
prokaryotic to eukaryotic cells (from
the second to the third level of evolution, according to my classification). One of the
members of this team is Bartolo Luque, with whom I have collaborated several
times and who has written comments in this blog. This article, which applies
algorithmic information theory to the history of living beings, contains two
significant figures that summarize their results.
The first figure, which I copy here, shows the variance of gene lengths as a function of their mean. The direction of evolution appears as a straight line, with prokaryotes (bacteria and archaea) in the lower right corner, followed by unicellular eukaryotes and fungi, then plants and invertebrates, and finally vertebrates in the upper right corner, where we can also distinguish between fish, birds, and primates. To obtain this figure, the team worked with the genomes of 33,627 species.
The second, even more significant figure,
represents the average length of proteins as a function of the average length
of the genes that represent them. The critical point, which corresponds to the
transition from the second to the third level of evolution, is clearly visible. At the time eukaryotes
appeared, proteins stopped growing, having reached their maximum length; but
genes continued to grow, with the appearance of introns and other forms of
regulatory DNA, allowing evolution to achieve many new results.
Another achievement of this article was the
estimation of the moment at which the critical point occurred: approximately
2.6 billion years ago, which falls significantly midway between the limits
usually considered most likely: between 2.9 and 2.3 billion years ago.
The article by Muro et al. says, regarding the
detected critical point:
As genes grew
beyond this point, the system’s available solutions grew enormously due to the
explosion of possible combinations. Finding one given solution, therefore,
becomes gradually easier.
This sentence demolishes one of the arguments often
put forward by proponents of intelligent design about the supposed
improbability of the emergence of proteins, calculated as the probability that a
given protein molecule would form spontaneously from a mixture of amino acids.
Their error, of course, is that it never forms spontaneously. This error was
first made by Lecomte de Noüy, whom I have mentioned in another
post.
Thematic Thread on Evolution: Previous Next
Manuel Alfonseca
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