The Origin of Life on Earth: How the First Life Forms Emerged

The origin of life on Earth, according to a widely accepted scientific hypothesis, first took place in the primeval ocean. Myriads of tiny clumps — protein bodies that arose at a particular stage in the development of matter from complex organic (protein) compounds — were carried by waves and sea currents from one place to another, gathering in quiet bays.

What are protein bodies and how did they form?

Protein bodies were living clumps that arose under the same natural laws but existed in somewhat different conditions, and so could not be entirely identical. This variability produced what Darwin once called natural selection: the clumps that survived were those whose metabolism ran more energetically, allowing other manifestations of life to emerge in them. The living clumps — protein bodies — gathered in quiet bays

A simple increase in the substance of a clump, resulting from assimilation prevailing over dissimilation, can be regarded as the growth of the primary organism (more on this: How the human organism is structured). Such growth, however, had to have limits, set by the physical conditions of the protein body's life and the particular features of its metabolism.

As life proceeded, the connection between organism and environment grew more complex, and the property of irritability developed as a result. This in turn led to the emergence of ever more complex systems of proteins. Different proteins began to perform different functions, and the protoplasm developed and became more elaborate.

At the same time, proteins acquired a relative stability and constancy, arising from the protein's own ability to rebuild, as it broke down, molecules similar to those that had been destroyed. Variability and heredity — the fundamental properties of living bodies — thus appeared already at the earliest stages of the development of life. The most important step in this development was the emergence of the simplest cellular forms, which undoubtedly reflected a higher stage of life and its continuous movement forward.

How does the study of microbes explain early life?

Modern single-celled creatures known as microorganisms, or microbes, help us understand how single-celled forms developed and how the first green (chlorophyll-bearing) plants arose. The study of microbes shows that metabolism proceeds differently in different species.

This, in turn, testifies to the exceptional diversity of their adaptation to varied conditions of existence. Indeed, how great is the variety of microbes in nature! Some require gaseous oxygen, while for others it is lethal; some live by breaking down organic matter and turning it into mineral matter, while others, on the contrary, create organic matter from mineral substances, and so on.

The microbes living in the soil are very diverse. Many microbes live in water, some at depths of several kilometres. There are microbes that assimilate organic matter at low temperatures on the snows of polar lands and in the permafrost zone, and there are others that live in hot springs.

What are fermentation microbes?

The simplest in terms of metabolism are the fermentation microbes, which are widespread in nature:

• milk turning sour,
• cabbage being pickled,
• bread dough being prepared with sourdough or yeast,
• wine being made from grape juice

— all of these are the result of the activity of fermentation microbes, which convert one organic substance into another.

When milk sours, for example, lactic acid bacteria convert the sugar contained in the milk into lactic acid. Both sugar and lactic acid are organic substances containing chemical energy, but sugar holds more of it than lactic acid. The activity of fermentation microbes consists precisely in their using this difference in energy, which is released as one organic substance passes into another.

There are serious grounds to believe that the most ancient organisms had a metabolism similar to that observed in fermentation microbes. More complex forms of metabolism evidently could not exist at that time, since there was no free oxygen in the atmosphere (it was bound up in various oxygen compounds).

Those more complex forms arose only later, when the composition of the atmosphere changed. The origin of life on Earth thus proceeded at the expense of the primeval organic substances formed in ancient bodies of water, and the energy of these substances was expended very slowly.

Yet however slowly the organic substances were consumed and however vast their reserves, the conditions of existence undoubtedly deteriorated, because new organic substances were formed in ever smaller and smaller quantities.

But life had already attained such diversity, complexity and capacity for adaptation that organisms which create organic matter from inorganic in the course of their metabolism were bound to arise — and indeed they did.

What are chemotrophic bacteria?

Two groups of such organisms are known today. The first are the so-called chemotrophic bacteria, which form organic matter using the energy released during the oxidation of sulphur, certain iron compounds and other inorganic substances. The oxidation occurs through the activity of the protoplasm of the vanishingly small and simple cells of these bacteria.

By the nature of their metabolism, such organisms gave rise to no complex forms, and could not do so: their life developed too monotonously and meagrely. In the course of their activity they proved highly "specialised" and could live only under strictly defined conditions. Chemotrophic bacteria, naturally, created no reserves of organic matter.

Over hundreds of millions of years they remained extremely low in organisation, and so their role in the development of life was negligible.

Yet, owing to the peculiarities of their activity, chemotrophic bacteria acted as "gatherers" of individual chemical elements and can be regarded as the creators of certain ore accumulations — for example, rocks containing iron, manganese and sulphur (more on this: Ferrous and non-ferrous metals and their ores).

How did chlorophyll-bearing plants advance life?

The other group of organisms — chlorophyll-bearing plants — gave rise to a mighty branch of life.

Chlorophyll is perhaps the most interesting of all substances in the whole organic world.

— C. Darwin. The organisms that existed at first lived on the energy of ready-made organic substances; chemotrophic and similar bacteria lived on the energy released as one mineral substance passed into another; whereas green, chlorophyll-bearing plants, using the energy of sunlight for their activity, began to create organic substances themselves.

An understanding of how this could have happened comes from studying how metabolism grew more complex in different organisms. It is known that a great role in the complication of metabolic processes is played by various inclusions found in the protoplasm and produced by its activity.

What are enzymes and why do they matter?

Among these inclusions, the so-called enzymes are especially important, as they accelerate and regulate metabolism in the organism. Therefore, the more complex an organism's metabolism, the more enzymes take part in its life processes.

In fermentation microbes, for example, very few enzymes are known. Lactic acid bacteria, for instance, have only one enzyme, which converts sugar into acid. The single-celled animal the amoeba already has many enzymes that participate in its nutrition, respiration, excretion and other life processes. In higher animals the enzymes are countless.

They perform diverse transformations of substances within the organism: they turn sugar into starch, starch into sugar, some kinds of proteins into others, and so on; they take part in the most complex physiological processes of the activity of the higher nervous system.

How do catalysts relate to enzymes?

Chemists have long known of substances which, while present during chemical processes, do not themselves change but accelerate the course of reactions. These substances are called catalysts, and the process is called catalysis. Adding a vanishingly small quantity of a particular catalyst to substances taking part in various chemical reactions can speed up the reaction many hundreds of times.

Enzymes perform a similar action within the organism. Catalysts act only under definite conditions and only with definite substances — that is, their action is strictly specific. Enzymes act in the organism in the same way. But while catalysts in chemistry can be very simple substances, enzymes are proteins (sometimes more complex, sometimes simpler), and their composition always includes a non-protein group of atoms, usually containing metallic elements.

The emergence and development of metabolism was evidently bound up with the phenomena of catalysis from the very beginning of life. The participation of simple catalysts, in the form of certain inorganic substances, in the chemical processes occurring in the primeval proteins led to the appearance of complex protein catalysts — that is, enzymes.

The interrelation of the various components of the protoplasm in the living cell took shape and grew more complex, evidently, through the participation of ever more diverse enzymes in the process of development. Reaching back into the depths of past ages, to the time when green plants arose, one cannot help linking the appearance of plants with the development of their enzymatic activity.

Besides enzymes, various other substances take part in the activity of organisms. Such are, for example, vitamins, diverse pigments (substances that give tissues one colour or another) and other inclusions. All of them were formed through the activity of the protoplasm. As a rule, they are useful adaptations that help the organism in its interaction with the environment.

The emergence of chlorophyll is undoubtedly connected with the activity of pigments. Probably at some point the role of some pigment grew more complex — perhaps in connection with a shortage of organic food: it not only began to help retain the energy of the sun's rays but also became a direct participant in chemical processes, playing the role of a kind of catalyst in the conversion of the active energy of sunlight into chemical energy — the energy of organic substances.

Thanks to the birth of chlorophyll, life advanced to a new and higher stage of existence, creating the conditions for its further development. The greatest investigator of the "mystery of the green leaf," K. A. Timiryazev, wrote of this victory of life:

Once, somewhere, a ray of the sun fell, but it did not fall on barren ground — it fell on... a grain of chlorophyll. Striking it, the ray went out, ceased to be light, but did not vanish. It was merely spent on inner work.

The further development and refinement of the primeval forms of protein bodies is covered in the article "How life arose in the ancient eras of the Earth".