Précis of epistemology/The origin and evolution of life and mind

The Birth of Venus, Adolph Hirémy-Hirschl

Evolution through natural selection

The theory of evolution through natural selection is based on three principles:

• Living beings transmit to their descendants hereditary characters.

• Small differences between the characters transmitted to the descendants and the characters inherited from the parents appear randomly.

• Living beings compete for access to the resources they need to live.

These three principles are sufficient to explain the evolution of all forms of life (Darwin 1859).

Each generation explores new hereditary possibilities a little different from the possibilities explored in the previous generation. If a living being is endowed with hereditary traits that favor it in the competition with its fellows, it will necessarily have a larger offspring, which will also be favored in the struggle for life. On the other hand, the unfavorable hereditary characters are little transmitted or not at all, and the lineages that transmit them eventually disappear. Natural selection therefore puts a pressure on the evolution of life forms which goes towards an increase in the desire to live. It retains the hereditary characters that help the most to live and lets others disappear.

The theory of evolution through natural selection can be applied to many fields. In principle, it can be used to solve any problem. It suffices to study generations of possibilities of solutions. Each generation is obtained from the previous one by randomly introducing small variations and selecting those that respond best, or least badly, to the problem. This is the method of genetic algorithms. Even learning by trial, error and success is very similar to evolution through natural selection. Opportunities are explored. We eliminate those that lead us to error and we retain those that make us succeed. We progress thus by modifying little by little what we learn, by evolving what helps us to live and by giving up the rest.

The molecules of heredity

Darwin and his or her successors did not explain the existence of heredity, why dogs do not make cats, they could only observe it. The existence of genes, mysteriously transmitted by the parents to their offspring, was postulated to account for the observations, but we did not know what they were, nor how they could determine the characters transmitted.

Genes are mainly DNA molecules (Avery, MacLeod and McCarty 1944, Watson and Crick 1953). Parents pass them on to their descendants by depositing them in their eggs. But how can molecules carry hereditary characters?

A DNA molecule is like a building plan for other molecules, RNAs and proteins. Proteins are both constituents of all organs, and tools that make it possible to build all the other molecules that living things must produce in order to survive, grow and reproduce. Moreover a DNA molecule is self-replicating, that is, it can serve as a blueprint for constructing an exact replica of itself. DNA molecules therefore make it possible to construct all the molecules that living things must produce. To understand how they determine hereditary traits, one must understand how proteins determine organ development and behavior.

The spontaneous generation of life in the primitive ocean

The theory of natural selection explains how life can evolve from primitive life forms. But where do these life forms come from?

The most likely answer is that life appeared spontaneously in the primitive ocean.

Autocatalytic networks

To grow and reproduce, one must be able to feed oneself, that is to say to draw from the environment molecules that serve as building materials to build all the molecules of the body, DNA, RNA, proteins and others. The molecules of a living being are constantly multiplied, for growth or to replace degraded molecules.

From a biochemical point of view, a living being is an autocatalytic network, ie a molecular system capable of catalyzing the production of its own molecules. A molecule catalyzes a chemical reaction if it is not consumed by the reaction and if its presence is necessary for the reaction to occur, or if it accelerates a reaction very slow in its absence. DNAs are catalysts for DNA and RNA synthesis reactions. RNAs are catalysts for the synthesis of proteins, DNAs, and RNAs. Some proteins are catalysts for the synthesis of DNA, RNA, other proteins and all molecules built by the body. In addition to DNA, RNA and proteins, living things can use any catalyst they find in their environment or they can produce.

Autocatalytic networks appear spontaneously as soon as the networks of chemical reactions are sufficiently complex and dense, that is to say that many molecules are capable of catalyzing numerous reactions (Kauffman 1993, 1995). The primitive ocean was a mixture of salt water and organic molecules. A molecule is organic when it is built on a carbon atom structure. Most molecules of living things are organic. The carbon atom is of all atoms the one that can form the most chemical bonds with other atoms. This is why organic molecules are very numerous, very diverse and often very reactive. We can do almost anything with organic molecules. They are of all shapes and for all uses. In the primitive ocean the networks of chemical reactions were thus complex and dense. Many organic molecules could react with each other and catalyze many reactions. Autocatalytic networks could therefore appear spontaneously.

Amphiphilic molecules and vesicles

To be alive one must have a skin or a membrane, which separates the inside from the outside. The physics of soapy water is sufficient to explain the spontaneous appearance of membranes. Soapy water contains amphiphilic molecules, ie elongated molecules, one end of which is hydrophilic and the other hydrophobic. A molecule is hydrophilic if it "prefers" to be in water rather than in oil, ie if it concentrates spontaneously more in water than in oil. It is hydrophobic, or lipophilic, in the opposite case. When amphiphilic molecules are dissolved in water, they spontaneously form many structures that can be very complex. In particular, they can form bilayers that fold into vesicles. A bilayer is like a wall, immersed in water, whose two faces are composed of hydrophilic ends, the hydrophobic ends having gathered inside the wall. A vesicle is a small bag full of water, immersed in water, and whose membrane is a bilayer.

A bilayer.The circles are the hydrophilic ends. The wavy lines are hydrophobic.

A vesicle

The cells of living beings are very elaborate vesicles. Their membrane is essentially a bilayer but it is much more complex than the membranes of the vesicles in soapy water. The interior of the cell especially is very different from its exterior, which is not the case of vesicles that are formed by shaking a soapy water.

The first living cells

Very many vesicles could be formed in the primitive ocean, as easily as today the foam of the sea. That such a vesicle can be the place of autocatalytic reactions is quite plausible. If its membrane is such that it lets in the small molecules necessary for the reproduction of the big ones, then we obtain, by the mere play of physical and chemical laws, an organism able to feed and grow. Of course vesicles are not always endowed with such a capacity, but if they are quite numerous and diversified enough, such an event is not completely improbable.

For a cell to be alive, it is not enough for it to be able to grow, it must still be able to reproduce itself. We can suppose that a sufficiently large vesicle can divide itself into smaller vesicles, or let protuberances come off.

If a vesicle is able to grow, feed, and reproduce, dividing or forming protuberances that come off, then it is a primitive living being. It has the essential property of living beings, the ability to reproduce when placed in an appropriate environment. We can then assume that such a vesicle is the ancestor of all living beings that exist today.

The origin of DNA

The first autocatalytic networks were not as elaborate as those of today, based on the very complex machinery of DNA, ribosomes, genetic code and proteins. But primitive living beings were able to evolve. Their autocatalytic networks could be modified by the incorporation of new molecules, exceptionally absorbed. Such changes are heritable, because once a molecule is incorporated into an autocatalytic network, it becomes able to reproduce. The conditions of evolution through natural selection are thus united: random inheritable variations and competition within a population for access to resources. The most successful primitive cells were those that reproduced best and they tended to dominate the population. We can then assume that these primitive living beings have evolved. Their rudimentary autocatalytic techniques have been perfected to the point of near perfection, namely the techniques of DNA replication and protein fabrication that have been possessed for billions of years by all living beings.

How to know if this theory is true?

We can not go back in time to see how the primitive ocean was and how it evolved. But we can find indirect testimonies. The past leaves traces in the present. If we have the right tools, theoretical and observational, we can deduce the past from the present. For example, today's autocatalytic techniques (DNA and company) have been like fossilized for billions of years, since they have not evolved, or little. They thus inform us about a very distant past. By combining this information with others, we can hope to go even further back in time. Laboratory experiments of prebiotic chemistry can provide valuable information.

Life is at home in the universe

This theory of spontaneous generation leads to a unified vision of matter and life. The appearance and evolution of life are conceived as necessary consequences of the dynamics of the universe. As soon as adequate conditions are met (organic molecules in abundance in liquid water, which supposes an adequate temperature), matter manifests its capacity to generate life. In a metaphorical way, we can say with Kauffman (1995) that living beings can feel at home in the universe, because matter is like the nourishing earth that gave us life.

Cooperation is more fundamental than competition

Life relies on cooperation between all molecules, and more generally, between all parts of an organism. When the parts stop working on the conservation of their whole, life disappears. The parts are alive because they are part of a living whole. A hand ceases to be a hand if it is separated from the body. If one applies the principle of Aristotle, that all parts of a living being vivify and are vivified by all the others, to molecular biology, one defines precisely autocatalytic networks.

Living beings are often competing against each other, but such a competition could not exist if there was not in the first place cooperation within the organisms. The struggle for life is a competition between systems that must cooperate internally. The winners are those who cooperate best. Natural selection retains the best forms of cooperation. It spontaneously discovers the best that matter is able to invent when it makes self-protective systems.

Rather than competition, living beings often have an interest in seeking cooperation among themselves, between members of the same species or between members of different species. Here again, natural selection favors those who cooperate best.

The infinite tolerance of life

Natural selection is sometimes conceived of as a sort of elimination of all that is not optimal, as if it were absolutely necessary to be the best to have the right to exist. But it is a mistake. Natural selection lets live whatever happens to live, optimal or not. The evolution of life is not the quest for the best, it is rather an exploration of all the possibilities of life. It reveals everything that can appear, it has no prejudices about what should be or not be. All forms of life are a priori welcome. Natural selection leaves weaknesses and imperfections alive. And even it requires their appearance, because the random novelties go in all directions, sometimes favorable, sometimes unfavorable, and because the weaknesses are also sometimes forces. Whether a hereditary trait is favorable or unfavorable depends on very many circumstances that may vary. This is why it is never possible to define a single optimum.

Nature is much more tolerant than us. When we say that what exists does not deserve to exist, Nature always says the opposite, since it lets what exists exist. From our point of view Nature does not sin by intolerance but by excess of tolerance. It lets live everything that wants to live, as soon as it happens to live, even the plague and all epidemics - epidemics are a natural manifestation of life, since microbes are alive - and all that horrifies us.

We exist because Nature allows us to exist. It is not very flattering, because it allows even the plague to exist. We would like to be at least a little better than the plague. The plague can not be better than it is, because it can not think of it, but we can.

The origin of mind

Life begins with the cooperation between molecules, autocatalytic networks, and unicellular organisms that they can build. It continues with multicellular organisms, the cooperation between living cells, and consequently between all the organs of a living being. It still continues with the cooperation between living beings.

Consciousness and will are also a form of cooperation within the living. An organism devoid of will is delivered to the circumstances. If its environment pushes it towards incompatible desires, its internal contradictions can only make it suffer and often perish. The conscious will, ie a centralized administration in the brain that imposes a minimum of internal coherence, is the appearance of a new form of cooperation, as if all parts of the body were able to agree on common objectives. Just as an intelligent state can increase the power of a human society, the conscious will increases the power of living, because it makes possible more cooperation between the parts of a living being.

Since natural selection promotes inner cooperation, the conscious will appeared and evolved as soon as living beings had the means to make it appear, that is, as soon as their brains were complex enough to develop a centralized administration.