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:. 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:. 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.
In the 18th century an Italian priest, Lazzaro Spallanzani , showed that fertilization of eggs by sperm was necessary for the reproduction of mammals.
Energy for the Origin of Life
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. Pouchet argued, defensibly, that life must somehow arise from nonliving matter; if not, how had life come about in the first place? 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. Toward the end of the 19th century, hypothesis 3 gained currency.
Swedish chemist Svante A. 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. However, they had extremely vague ideas about how this might be accomplished. 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.
- Hypotheses of origins.
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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.
Dark Matter and the Origin of Life
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 organism to organism, 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. They are largely gaseous, with atmospheres composed principally of hydrogen and helium.
Science for a Complex World
Methane , ammonia , neon, and water have been detected in smaller quantities. This circumstance very strongly suggests that the massive Jovian planets formed from material of typical cosmic composition. Because they are so far from the Sun, their upper atmospheres are very cold.
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Atoms in the upper atmospheres of the massive, cold Jovian planets cannot now escape from their gravitational fields, and escape was probably difficult even during planetary formation. Earth and the other planets of the inner solar system , however, are much less massive, and most have hotter upper atmospheres.
 Origins of Life: A Problem for Physics
Most likely the atoms carbon, nitrogen, and oxygen were present on the early Earth, not in the forms of CO 2 carbon dioxide , N 2 , and O 2 as they are today but rather as their fully saturated hydrides: methane, ammonia, and water. The presence of large quantities of reduced hydrogen-rich minerals, such as uraninite and pyrite , that were exposed to the ancient atmosphere in sediments formed over two billion years ago implies that atmospheric conditions then were considerably less oxidizing than they are today.
Image credit: M. Cappellari and the Sloan Digital Sky Survey. All of these forms of normal matter -- or matter originally made of the same things we are: protons, neutrons and electrons -- do in fact contribute to what's there, with gas and plasma in particular contributing more than even stars to. For the rest of it, we need a new form of matter that isn't just different from protons, neutrons and electrons, but that doesn't match up with any of the known particles in the Standard Model. Mahdavi et al. Dawson et al.
Allen Stanford University bottom right. These colliding galaxy clusters show a clear separation between the normal matter in pink and the gravitational effects in blue. But what might surprise you is that we don't just need dark matter to explain galactic rotation, cluster motions and collisions, but to explain the origin of life itself!
To understand why, all you need to remember is that the Universe began from a hot, dense state -- the hot Big Bang -- where everything started off as a mostly uniform sea of individual, free, high-energy particles.
dialadethsara.gq As the Universe expands and cools, we can form protons, neutrons, and the lightest nuclei hydrogen, deuterium, helium and a trace amount of lithium , but nothing else. It isn't until tens or even hundreds of millions of years later that matter will collapse into dense enough regions to form stars and what will eventually become galaxies.
All of this will happen just fine, albeit differently in detail, whether there were plenty of dark matter or none at all. But in order to make the elements necessary for life in great abundance -- elements like carbon, oxygen, nitrogen, phosphorous and sulphur -- they need to be forged in the cores of the most massive stars in the Universe.