The Explosion of Life Forms

Living Beings and Morphology

• Éditeur: Wiley-ISTE
• Année: 2021
• ISBN:
  9781119818427
• Édition: 1
• Sujet: Biological Sciences
• Sous-sujet: Biology

One of the essential characteristics of living beings is the explosion of variety in their forms that is intrinsically linked to the diversity of the environments they have adapted to.

This book, the result of collaboration between international specialists, analyzes the multiplicity of these morphologies. It explores the origin of forms, their role in defining living things, and the relationship between form and function. It exposes the role of genes and epigenetics and examines the forms of bacteria, protists and plants. The Explosion of Life Forms also studies the memory of animals and their sensory processes, the forms of robots (built in the image of living things), and medical technologies aimed at restoring damaged living forms. Finally, this work questions a common principle of construction in the diversity of forms, as well as the idea of an abandonment of the form, a possible hidden defect of some modern philosophies.

1
Possible Traces and Clues of Early Life Forms

Marie-Christine MAUREL

ISYEB-CNRS-MNHN, Sorbonne University, Paris, France

“The pivotal question of the living ... is that of morphogenesis.”

Claude Bernard, 1878

1.1. Introduction

Since its formation 4.5 billion years ago, the Earth has occupied a unique place in the solar system. The presence of liquid water, the first cradle of life, on 70% its surface is one of the major features of its appearance: 90% of life’s history has taken place in water and we still ask ourselves numerous questions about its formation today.

Around 3.85 billion years ago, mineral matter and organic matter intertwined to produce, by chemical reactions, the amino acids of proteins, the constituents of the nucleic acids RNA and DNA (nitrogenous bases, sugars, phosphates), and all sorts of blocks that contributed to the design of the matrix of living things. Less than a billion years after the accretion of our planet, the elementary building blocks of biological molecules were thus formed on the primitive Earth. Today, life is omnipresent, covering the entire planet and its systems, at all latitudes, including our skin and our digestive tract, inhabited by thousands of bacteria species.

There is a very wide variety of shapes, from rod-shaped bacilli to spherical shells, as well as helical, spiral or star-shaped structures, to name but a few of the forms observed in microscopic bacteria, that are living either in isolation, in filamentous association, in symbiosis, etc. The diversity of shapes and sizes is also observed in viral particles (Adriaenssens et al. 2018) and in multicellular organisms such as humans, snails, ferns, geckos, etc.

And diversity also manifests itself over time: nowadays, we no longer find pithecanthropes, lepidodendrons, tyrannosaurs, ammonite trilobites, etc.

Are the varieties observed today the only ones possible? Are there other paths, other formats, other modes that have not (yet) been explored by nature? Or by our own understanding? Or by our technological limits?

The total number of living species is estimated at 10¹², and only 10⁵ of these have been identified to date (Locey and Lennon 2016). We only know 2–10% of the species that exist today, which represents 1/1,000 of the species that have existed for 3.85 billion years (Mora et al. 2011).

These data alone justify the weakness of our generalizations.

1.2. Have “things” always been as they are today?

What prebiotic chemical reactions can reasonably be simulated in the laboratory? The attributes of today’s living organisms (structures, perennial and hereditary metabolic pathways) have passed through billions of years of planetary, geological and environmental events. Natural selection, by not retaining what was becoming inadequate, has amplified new attempts and taken the place of what was left vacant by recent extinctions, thus favoring innovation. This is how new species settle into diversity.

It is worth recalling that Jean-Baptiste Lamarck, in Philosophie zoologique (1809), was the first to develop a physical theory of living beings, that is to say, an organization of the subject driven by a series of physical processes. He considered that the simplest beings were formed in an appropriate environment in response to physical-chemical laws. According to Lamarck, the living beings resulting from this spontaneous generation adapted to the environment and thus became more “complex”.

The invention of the microscope in the 21st Century revealed that all living organisms are made up of cells of different shapes. Since then, the cell has been regarded as the fundamental structural and functional biological unit of all living beings.

A student of Justus von Liebig, Moritz Traube, developed an “artificial mineral cell” from copper and potassium ferrocyanide in 1867. He observed the growth and budding of structures that resembled cell-like forms like those observed by Robert Hooke in 1665.

Then, Stéphane Leduc, inventor of the term “synthetic biology” (1912), declared at the beginning of the 20th Century, despite the craze for chemistry at the time: “Why is it less acceptable to try to find out how to make a cell than to make a molecule?”

Later, in the 1920s, the Soviet biochemist Alexander Oparin and the English geneticist John Burdon Sanderson Haldane proposed that elementary bricks of life, formed from gaseous elements in the atmosphere, were deposited in the primitive ocean, creating a “prebiotic soup” rich in assorted molecules that, when assembled, formed protocells or coacervates. From then on, all sorts of microspheres, micelles, protobionts and other models of single cells were imagined to represent simple compartments.

In order to “come alive”, the first processes were therefore able to take place in micro-environments that were capable of keeping the different components close to each other, thus promoting their interactions. Initially, this could be in the hollow of a rock or on clay dust, then in tiny organic vesicles, a kind of small bag that fills with molecules until it splits in two.

Coacervates were synthesized in a laboratory in the 1930s by Bungenberg de Jong. Proteinoids (Fox 1988), microspheres, marigranules and other compartmentalizing structures were obtained in order to mimic early cell forms.

In 1951, the young biochemist Boris Pavlovitch Belooussov, who worked in the Biophysics Laboratory of the Ministry of Health of the USSR, wanted to create an inorganic reaction that was similar to the energy-producing reaction in all aerobic organisms. Such a pathway, called the “Krebs cycle” (or “citric acid cycle”, abundant in lemons), works in living cells to break down sugars and produce energy. Belooussov mixed bromate ions BrO3^- with citric acid C₆H₈O₇ in the presence of ceric ions Ce⁴⁺ in an acidic medium. He hoped to reduce cerium (4⁺) to cerium (3⁺), which would have caused a discoloration in the solution. However, his observations were quite different, since he noticed a periodic succession of colors and discolorations of the reaction medium: Belooussov had just discovered the first oscillating reaction by chance. Ten years later, Anatol Zhabotinsky confirmed these observations, which had previously received little credit, and in turn described these oscillating, colored and “budding” waves, which earned the two authors the Lenin Prize for the now famous Beloousov-Zhabotinsky reaction. These observations inspired Heinz von Foerster’s (1961) and Henri Atlan’s (1972) theories on selforganization, Francisco Varela’s autopoiesis (1974) and Ilya Prigogine’s (1947) “thermodynamics far from equilibrium”, with related speculations on the origins of life.

1.3. Fossil traces?

Despite laboratory experiments and increasingly sophisticated techniques, we are faced with the absence of traces of the past, irrefutable “evidence” of the first moments. Indeed, down-to-earth, strictly geological considerations make it difficult to explore morphological evidence of the past. Volcanoes, tectonic plates, metamorphism and massive meteorite bombardments occurr...