How did the first cells develop? Many people think that scientists claim that chemicals randomly mixed together to form living cells (left side of image below).
However, that is not quite what scientists are saying. Rather, they propose a slow, step-by-step process (right side of image below.)
These steps have been demonstrated in a wide variety of experiments.
Organic molecules form spontaneously
This has been observed happening in many different environments, in different ways.
* in oceans
* in clays
* asteroids and meteors, where chemicals are bombarded with ultra-violet light from the sun.
* in nebulas (stellar gas clouds)
In March 2015, NASA scientists reported that…complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites.
“NASA Ames Reproduces the Building Blocks of Life in Laboratory”.
Tons of these organic molecules have rained down upon Earth, and other worlds, for billions of years.
Under some conditions organic molecules replicate
Under the right conditions, when these replicating molecules are in a protected environment, they can replicate.
Random replications, trillions of times
Random changes may happen in billions of different ways, on different parts of the world, per hour, over millions of years.
At any given moment, over the surface of a planet, trillions of chemical reactions are happening per second, every second, for billions of years.
Molecules get trapped inside vesicle
At some point, some of these reactions became more efficient, when trapped inside other molecules (thus forming an early “cell.”)
We do not know the exact way in which the first living cells developed here on Earth. But we now now many possible paths that organic molecules can form, and many possible paths they they could replicate.
Exploring Life’s Origins App
More than one possible way life may have started
Scientists have discovered many pathways by which organic molecules could create living cells.
<h3<We don’t know which of these pathways happened here; perhaps the universe has many ways that life can arise.
1. Hydrogen cyanide path
“…everything necessary for life to evolve could have done so from just hydrogen sulfide, hydrogen cyanide and ultraviolet light – and that those building blocks could have all existed at the same time. In their paper, they report that using just those three basic ingredients they were able to produce more than 50 nucleic acids: precursors to DNA and RNA molecules. “
“They note that early meteorites carried with them ingredients that would react with nitrogen already in the atmosphere, producing a lot of hydrogen cyanide. By dissolving in water, it could have very easily come into contact with hydrogen sulfide, while being exposed to ultraviolet light from the sun. And that, they claim, would have been all that was needed to get things going.”
“..We show that precursors of ribo-nucleotides, amino acids and lipids can all be derived by the reductive homologation of hydrogen cyanide and some of its derivatives, and thus that all the cellular subsystems could have arisen simultaneously through common chemistry
2. Deep Sea Vent Theory—Hydrogen saturated, heated, fluids from hydrothermal vents on the ocean floor mix with carbon dioxide laden water. Continued chemical energy from the interactions sustains processes that produce simple organic molecules.
3. Spontaneous Formation of Small Peptides from Amino Acids: Sidney Fox demonstrated that the conversions could occur on their own.
4. Eigen and Schuster argue that some molecules, possibly RNA, can serve as an information storing system that brings about the formation of other information storing systems, or a kind of replication.
5. Günter Wächtershäuser argues that some compounds come with inboard energy sources like iron sulfides that could release energy and synthesize simply organic molecules. His experiments produced small amounts of dipeptides and tripeptides.
6. Radioactive beach hypothesis: radioactive elements such as uranium may have concentrated on beaches and become building blocks for life by energizing amino acids, sugars from acetronitrile in water.
7. The RNA, “genes first”, world It has been argued that short RNA molecules could have formed on their own. Cell membranes could have formed from protein-like molecules in heated water. Chemical reactions in clay or on pyrites could have initiated self-replication.
8. “Metabolism first” models: iron-sulfur world and others. Some theories argue that metabolic processes started first, then self-replication.
9. Clay theory – Complex organic molecules could have arisen from non-organic replicators such as silicate crystals. It has even been reported that the crystals can transfer information from mother to daughter crystals.
10. Gold’s “Deep-hot biosphere” model Gold argues that life originated miles below the surface of the earth. Microbial life has been found there. And it may be present on other planets.
Left handed versus right handed molecules
* explanation of handedness, in general *
* explanation of handedness, in molecules *
* all life on Earth has the same handedness; reason(s) unknown *
Homochirality: The right or left handedness of organic molecules may be explained by the origin of compounds in space.
Did abiogenesis happen just once, or many times?
Since all lifeforms on Earth today are similar on a molecular level (DNA), suggesting a common origin. This appears to imply at least one of the following:
- Immediately after the first spontaneous abiogenesis, environmental conditions on Earth changed dramatically, making a repeat impossible.
- The first organisms that arose consumed any subsequent organisms that came into being.
This was suggested by Alexander Oparin, but I find it very difficult to believe since it would have to happen everywhere on Earth.
Also, most single-celled organisms found today do not consume other organisms, and even where they do that does not usually lead to their complete extinction.
- The common origin theory is in fact false:
Despite the similarities between organisms, abiogenesis did occur multiple times in the same way.
- Abiogenesis happened more than once, but only the descendants of one occurrence survived till today.
- Spontaneous abiogenesis never occurred on Earth after all because the conditions never allowed it;
instead, a proto-organism arrived on Earth from a planet where the conditions do (Panspermia).
- The standard theory of geological history is wrong
(i.e. the Earth was in a “fertile” state for much longer before the eventual origin of life than commonly thought,
due to some unknown mechanism causing radiometric dating to give wrong results).
Abiogenesis FAQs: Articles on the Origin of Life
A shadow biosphere?
Could there be a shadow biosphere here on Earth?
These researchers believe life may exist in more than one form on Earth: standard life – like ours – and “weird life”, as they term the conjectured inhabitants of the shadow biosphere.
All the micro-organisms that we have detected on Earth to date have had a biology like our own: proteins made up of a maximum of 20 amino acids and a DNA genetic code made out of only four chemical bases: adenine, cytosine, guanine and thymine,” says Cleland. “Yet there are up to 100 amino acids in nature and at least a dozen bases. These could easily have combined in the remote past to create lifeforms with a very different biochemistry to our own. More to the point, some may still exist in corners of the planet.”
Science’s failure to date to spot this weird life may seem puzzling. The natural history of our planet has been scrupulously studied and analysed by scientists, so how could a whole new type of life, albeit a microbial one, have been missed? Cleland has an answer. The methods we use to detect micro-organisms today are based entirely on our own biochemistry and are therefore incapable of spotting shadow microbes, she argues. A sample of weird microbial life would simply not trigger responses to biochemists’ probes and would end up being thrown out with the rubbish.
That is why unexplained phenomena like desert varnish are important, she says, because they might provide us with clues about the shadow biosphere. We may have failed to detect the source of desert varnish for the simple reason that it is the handiwork of weird microbes which generate energy by oxidising minerals, leaving deposits behind them.
The idea of the shadow biosphere is also controversial and is challenged by several other scientists.
Equivalent term: biological dark matter
‘Dark Matter’ in Biology
Paradigms and Biological ‘Dark Matter’
‘Dark Matter’ in Biology: Great Expectations and Biological Limits
A Dark Shadow Biosphere with Unorthodox Orthogonality?
A Dark Shadow Biosphere with Unorthodox Orthogonality?
Does ‘Dark’ Biology Have Its CHARMs?