https://www.quora.com/Is-abiogenesi...-solid-theories-concerning-the-origin-of-life
by
Krister Sundelin
E-learning Producer (2020-present)
Updated September 22 ·
Upvoted by
Keith Robison
,
Ph.D. In Molecular&Cellular Biology; in Biopharma since 1996
Q: Is abiogenesis mostly considered fact by scientists, or is it merely a belief held by atheists due to lack of solid theories concerning the origin of life?
A: Atheists have nothing to do with it. Atheists only don’t believe in gods.
Abiogenesis is a serious field of study in biochemistry, biology, and genetics, in which those scientists
hypothesise different ways of how you can get life from non-life and test their ideas.
The hypotheses are not a complete path from non-life to life. Scientists don’t start with a bunch of chemicals and shake them and see if there was life coming out of it. Instead, there are much mote specific, like how can you get something like a cell membrane from nothing but lipids, how can amino-acids come to be naturally, or how could RNA form without a template to start with.
So the field of abiogenesis is much more like building and testing rungs on a ladder and how they could be combined to a ladder, rather than showing the final ladder. It is much more showing how you can get life from non-life, and very much step by step; not necessarily how we got life from non-life.
It’s very hard to deduce how we got life from non-life. The difficulty is that scientists in the field are working backwards, not the original chemistry. It happened 3.5 billion years ago, and very few clues remain.
There are some clues in virology, some clues in genetics, some in geology, and so on, but they are like hints at individual rungs on the ladder, not the full story. For instance, there are some 350 genes that are common to all living things. There is some emerging evidence that proto-viruses may be the first living thing – previously, it was thought that viruses evolved later as simplified cells. There is the Miller-Urey experiment that shows that amino-acids can form in primitive conditions. There are even finds from comets and asteroids which show amino-acids in space, and quite recently even entire proteins (although short ones) in asteroids.
But it is quite another challenge to assemble these clues and put together a complete story of how life started on Earth. There are three main hypotheses, the RNA World, the Protein World, and the Lipid World.
The RNA World hypothesis. RNA is the “little sister” of DNA: one half of the ladder, but made up by basically the same chemistry (it has a different sugar molecule in the backbone – ribose instead of deoxyribose). It can form spontaneously under the right conditions, and also self-replicate.
There is support for this idea in for instance that many viruses consists only of an RNA strain in a protein shell.
One interesting sub-hypothesis to the RNA world hypothesis is that some forms of clay crystals – which themselves self-replicate – could act as catalyst or “scaffolding” for the formation of RNA.
The Protein World hypothesis. We already know from Miller-Urey and the Stardust probe that amino-acids, the building blocks of proteins, can form naturally. Amino-acids can also polymerise into short polypeptides naturally in the right conditions. This is supported by a quite recent find of the protein hemolithin in a meteorite from Algeria. Some proteins can self-replicate too – there are the nasty prions of Creutzfeldt-Jakob disease (also known as mad cow disease), which is caused by misfolded proteins that catalyse other proteins to misfold as well.
Again, there is some support for this idea from viruses, specifically their injector mechanisms and shells.
The Protein World and even more so the RNA World provides the backbone for the “replication first” model, which suggests that if you have replication, you can have something similar to evolution but in chemistry: chemistry complexes which work can replicate and evolve, while chemistry complexes which do not work will not replicate.
The Lipid World hypothesis. Lipids, for instance fatty acids, are actually pretty simple molecules, but they have the ability to form bubbles (micelles and liposomes), which also can self-replicate. The great advantage of a lipid bubble is that it separates the world into two parts: inside and outside. Molecules trapped inside a liposome cannot be washed away, so they are always in contact with other molecules.
It also creates an energy potential between inside and outside, which in turn allows for more complex chemistry to happen. This is the basis for the “metabolism first” model, which suggests that for the chemistry of life to happen, you need energy pathways or metabolism first, and for that to happen, there must be an energy potential. The Lipid World provides this energy potential between the inside and the outside.
All of the above. It may be – and is pretty likely – that all of the above happened. The question is rather in which order they combined, or if two or more happened at the same time.
Viruses could indicate that the protein world and the RNA world combined first, which may have been an advantage compared to separate protein and RNA complexes. These were then enveloped in lipid bubbles, which gave the entire complex a survival advantage compared to protein-RNA complexes and separate lipid bubbles in the wild. This is one example of a “replication first” model.
Another variant could be that liposomes and proteins combined first, and that they in turn absorbed RNA to catalyse protein replication. This would be an example of a “metabolism first” model.
Although the field of abiogenesis is yet incomplete, there are several hypothetical pathways and scenarios for the process of simple chemistry becoming complex chemistry becoming chemistry complexes becoming what could resemble “life as we know it”. The rungs of the ladder are mostly there, but we don’t know the details.
“But Krister”, I hear you say, “it really sounds as if scientists assume that abiogenesis happened. Isn’t that unscientific?”
Well, what scientists don’t assume is that magic or gods were involved. That would be unscientific, and also unfalsifiable and untestable due to the alleged nature of the supernatural (i.e. beyond the natural world). Science does assume that there is a natural process behind everything. And that includes the origin of life.
What abiogenesis is about is trying to pry out how it could and can happen naturally from the clues that are left.
