The origin of life:

✨Hypotheses of origins

Perhaps the most fundamental and at the same time the least understood biological problem is the origin of life. 

It is central to many scientific and philosophical problems and to any consideration of extraterrestrial life. Most of the hypotheses of the origin of life will fall into one of four categories:

✨The origin of life is a result of a supernatural event—that is, one irretrievably beyond the descriptive powers of physics, chemistry, and other science.

Life, particularly simple forms, spontaneously and readily arises from nonliving matter in short periods of time, today as in the past.

Life is coeternal with matter and has no beginning; life arrived on Earth at the time of Earth’s origin or shortly thereafter.

Life arose on the early Earth by a series of progressive chemical reactions. Such reactions may have been likely or may have required one or more highly improbable chemical events.

✨Michelangelo: The Creation of Adam

Hypothesis 1, the traditional contention of theology and some philosophy, is in its most general form not inconsistent with contemporary scientific knowledge, although scientific knowledge is inconsistent with a literal interpretation of the biblical accounts given in chapters 1 and 2 of Genesis and in other religious writings. Hypothesis 2 (not of course inconsistent with 1) was the prevailing opinion for centuries. A typical 17th-century view follows:

👉Note :doubt whether, in cheese and timber, worms are generated, or, if beetles and wasps, in cow’s dung, or if butterflies, locusts, shellfish, snails, eels, and suchlike be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by the formative power disposed.

 To question this is to question reason, sense, and experience. If he doubts of this, let him go to Egypt, and there he will find the fields swarming with mice begot of the mud of the Nylus [Nile], to the great calamity of the inhabitants.

It was not until the Renaissance, with its burgeoning interest in anatomy, that such spontaneous generation of animals from putrefying matter was deemed impossible. During the mid-17th century the British physiologist William Harvey, in the course of his studies on the reproduction and development of the king’s deer, discovered that every animal comes from an egg. An Italian biologist, Francesco Redi, established in the latter part of the 17th century that the maggots in meat came from flies’ eggs, deposited on the meat.

 In the 18th century an Italian priest, Lazzaro Spallanzani, showed that fertilization of eggs by sperm was necessary for the reproduction of mammals. Yet the idea of spontaneous generation died hard. Even though it was clear that large animals developed from fertile eggs, there was still hope that smaller beings, microorganisms, spontaneously generated from debris. Many felt it was obvious that the ubiquitous microscopic creatures generated continually from inorganic matter.

Maggots were prevented from developing on meat by covering it with a flyproof screen. Yet grape juice could not be kept from fermenting by putting over it any netting whatever. Spontaneous generation was the subject of a great controversy between the famous French bacteriologists Louis Pasteur and Félix-Archimède Pouchet in the 1850s. Pasteur triumphantly showed that even the most minute creatures came from “germs” that floated downward in the air, but that they could be impeded from access to foodstuffs by suitable filtration. Pouchet argued, defensibly, that life must somehow arise from nonliving matter; if not, how had life come about in the first place?

Pasteur’s experimental results were definitive: life does not spontaneously appear from nonliving matter. American historian James Strick reviewed the controversies of the late 19th century between evolutionists who supported the idea of “life from non-life” and their responses to Pasteur’s religious view that only the Deity can make life.

 The microbiological certainty that life always comes from preexisting life in the form of cells inhibited many post-Pasteur scientists from discussions of the origin of life at all. Many were, and still are, reluctant to offend religious sentiment by probing this provocative subject. But the legitimate issues of life’s origin and its relation to religious and scientific thought raised by Strick and other authors, such as the Australian Reg Morrison, persist today and will continue to engender debate.

Toward the end of the 19th century, hypothesis 3 gained currency. Swedish chemist Svante A. Arrhenius suggested that life on Earth arose from “panspermia,” microscopic spores that wafted through space from planet to planet or solar system to solar system by radiation pressure. This idea,

 of course, avoids rather than solves the problem of the origin of life. It seems extremely unlikely that any live organism could be transported to Earth over interplanetary or, worse yet, interstellar distances without being killed by the combined effects of cold, desiccation in a vacuum, and radiation.

Although English naturalist Charles Darwin did not commit himself on the origin of life, others subscribed to hypothesis 4 more resolutely. The famous British biologist T.H. Huxley in his book Protoplasm: The Physical Basis of Life (1869) and the British physicist John Tyndall in his “Belfast Address” of 1874 both asserted that life could be generated from inorganic chemicals. However, they had extremely vague ideas about how this might be accomplished. 

The very phrase “organic molecule” implied, especially then, a class of chemicals uniquely of biological origin. Despite the fact that urea and other organic (carbon-hydrogen) molecules had been routinely produced from inorganic chemicals since 1828, the term organic meant “from life” to many scientists and still does. In the following discussion the word organic implies no necessary biological origin. 

The origin-of-life problem largely reduces to determination of an organic, nonbiological source of certain processes such as the identity maintained by metabolism, growth, and reproduction (i.e., autopoiesis).

Darwin’s attitude was: “It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter.” The two problems are in fact curiously connected. Indeed, modern astrophysicists do think about the origin of matter. The evidence is convincing that thermonuclear reactions, either in stellar interiors or in supernova explosions, generate all the chemical elements of the periodic table more massive than hydrogen and helium. 

Supernova explosions and stellar winds then distribute the elements into the interstellar medium, from which subsequent generations of stars and planets form. These thermonuclear processes are frequent and well-documented. Some thermonuclear reactions are more probable than others. These facts lead to the idea that a certain cosmic distribution of the major elements occurs throughout the universe. Some atoms of biological interest, their relative numerical abundances in the universe as a whole, on Earth, and in living organisms are listed in the table. Even though elemental composition varies from star to star, from place to place on Earth, and from organismo oganism, these comparisons are instructive: the composition of life is intermediate between the average composition of the universe and the average composition of Earth. Ninety-nine percent of the mass both of the universe and of life is made of six atoms: hydrogen (H), helium (He), carbon (C), nitrogen (N), oxygen (O), and neon (Ne). Might not life on Earth have arisen when Earth’s chemical composition was closer to the average cosmic composition and before subsequent events changed Earth’s gross chemical composition?