If life formed on earth by natural laws, why can't we observe the formation of life from matter today? Is it because this is a rare phenomenon? It seems just after formation of earth life formed on earth and around 2-3 billion years has passed. It must be enough time for life to form several times from scratch.
The first life was probably so fragile and simple we would likely not even recognise it if it did appear. We don't know exactly how, or where, life first appeared but nearly all theories suggest simple collections of enzymes in protected environments that slowly gather the mechanisms needed for free-living life over time.
This works in the ancient world because there is no competition so even the simplest and most inefficient replicator is able to reproduce, spread, and advance. Today, any new life would have to compete with existing life which has had billions of years to evolve into highly efficient forms. A new abiogenesis event would simply pass unnoticed, a brief flicker quickly snuffed out by modern life.
Well.. consider this... once life already exist... and a new life form spontaneous came into came into existence, how likely is the newcomer going to be able to compete with the current contender, who now has time to develop all sorts of abilities from being in an arms race between members of its own group. (A bit like how likely is a newly arrive group of cave men going up against a nation armed with Abrams tanks)
Also note... the first group of earthly life started polluting the sea and atmosphere with a highly toxic and damaging chemical... oxygen (the Great Oxygenation Event @The Oxygen Holocaust 2.5 billion year ago). And oxygen damages reactive chemical group required to catalyse chemical reactions and oxygen also pretty much destroyed the rich chemical soup that life first arose. Even today there are many groups bacteria and archea that cannot tolerate oxygen.
tldr once life evolved, it destroyed the environment that allowed it to arise in the first place. Environmental destruction is a long held tradition of earthy life.
Also the free for all chemical buffet that was on early earth... the first life to emerge would have would have eaten it all up. Leaving nothing, no building block for later life to to emerge form.
For a start you need an aseptic environment for a new, defenseless kind of life. All water containing environments on our planet conducive to new life are septic.
Primitive life forms are rare and less competitively robust and performant. The new life forms would be like worms that can't dig, and the current microbes would be like chickens, all mixed together in the same room.
I you had a stable warm C/H/N/O rich cave/seafloor environment which stayed protected for millions of years, you might get the first kinds of organism appear there. They would have 4 billion years less genetic sophistication than current microbes and funghi for attack, defense, fast adaption, diversification, speciation, pH tolerance, mobility, food selection, digestive processes, which today pervade and feed off every environment on earth.
The new life which took very long to evolve has to defend itself agains a lot of species of funghi, procariotes, eukariotes, animals and plants, while it is in the cave and after it leaves it.
If you brought back all weird jellyfish type ediacaran species today, and put boat loads of them into the sea, the fish and advanced jellyfish would decimate them and consume them. their primitive design makes them only food sources.
The advancement of a modern procariote compared to an instar life-form as the difference between 1970's calculator and an iphone in terms of performance.
The answer may be that life did not form on Earth. There is not even the slightest hint of evidence that life formed on Earth. The known facts about early lifeforms on Earth from fossils and everything we know about geology of the early Earth and the Early solar system is at odds with abiogenesis having occurred on Earth. One of the arguments against abiogenesis on Earth is precisely the question asked.
The problem is not just that we don't see life forming in a natural environment, as this can be easily explained away by invoking competition from other life forms. Also one can note that the conditions on Earth today are different from that of the early Earth (oxygen is poisonous to early life forms). A more fundamental problem is that we can't reproduce some of the fundamental chemistry needed for abiogenesis in the lab under conditions that have existed on our planet during the time life could have evolved.
E.g. the synthesis of ribose, which is the backbone of RNA from inorganic chemicals does not seem to be possible under Earthlike conditions. A recent experiment demonstrated that under intense ultraviolet irradiation of water, methanol and ammonia under cryogenic conditions at extremely low pressures one can form ribose in the lab. Similarly, we cannot get amino acids to form under Earth-like conditions, we again need to resort to irradiating interstellar ice analogues in near vacuum conditions.
We also need to consider how can we end up with only left or right handed molecules of each type as found in Nature. It has been demonstrated that replicating systems where both variants are presents, will lead to an end state where only one type will survive. But we then get domain formation where in different disconnected farts the symmetry will be broken in a random way. This may be good enough to explain why we only have left-handed life on Earth today.
An alternative idea is to invoke the fact that the Weak interaction does not respect parity symmetry. Now the Weak interaction is too weak to be relevant for molecular dynamics. But in radioactive beta-decay, this interaction plays a dominant role, beta-radiation consist of only left-handed electrons. One can then postulate that abiogenesis happened in an environment where there was irradiation by cosmic beta radiation, which would lead to an asymmetry in the breakup of chiral molecules, which would then be further amplified by auto-catalytic processes. This is the Vester-Ulbrict hypothesis, which was recently been successfully put to the test in the lab.
Another argument against abiogenesis on Earth is that the required far from thermal equilibrium conditions never existed on Earth, while they do exist in objects like comets and proto-planets. This is more general argument than the examples given above. Basically, what you want is to get to a collection of self replicating molecular machines from their simple building blocks, without there being any machines to build them from scratch.
Molecules in living organisms are synthesized using extremely complex molecular machinery such as the ribosome. Without such machinery, the natural environment has to play the role of the agents involved in the construction. What is needed are both far from thermal equilibrium conditions and places where half build, unstable molecules can be metastable, allowing the complete molecule to appear. This suggest that cryogenic conditions should play a role here.
We can also note here that life on Earth exists in a compartmentalized form, all the chemistry happens inside a lipid membrane (a cell). Many of the essential molecules would not be stable in a hypothetical environment that could have existed in Earth's geological history. Then just like life today exists mainly in conditions that were hostile to early life forms (the oxygen in our atmosphere would kill them), similarly it may well be the case that conditions on Earth have always been hostile to processes that could lead to abiogenesis.
The molecules essential for life as we know it may have evolved without any compartmentalization in the interior of comets or proto-planets (this would qualify as life in the sense of an auto-catalytic process that is capable of evolving), compartmentalized lifeforms could have arisen later there.
A different argument comes from the conditions in the early solar system and what we know about life on Earth from the geological record. The earliest evidence for life on Earth dates back to 3.7 billion years ago. These are stromatolites, which then shows that 3.7 billion years ago there were very sophisticated life forms on Earth.
Petty much all of the enormously complex molecular machinery found in modern cells had to be present already in the bacteria that formed these stromatolites. Abiogenesis would thus have had to occur quite some time earlier. However, from 4.1 to 3.8 billion years ago, the inner solar system was subject to what is called the Late Heavy Bombardment (LHB). During this period the conditions on Earth were rather inhospitable for life. Whether life could have survived impacts so large that oceans on Earth would have evaporated is a matter of ongoing debate.
One has to note here that the people who argue that life did evolve on Earth during this period, tend to favor LHB scenarios that are one the extreme limits of the geological scenarios, one then assumes that it was not that much of a big deal, e.g. the largest impacts could have been smaller than what most geologist think. However, recent geological evidence points in the opposition direction. It has recently been found that the size of the impactor that formed Mare Imbrium on the moon was enormously larger than was previously estimated. The impactor is now estimated to have had a diameter of 250 km.
Taking into account that the Earth is a much larger target than the Moon, so it would have been hit more frequently than the Moon and the largest objects to hit the Earth would also be larger than the largest ones to hit the Moon, the idea that perhaps the LHB was not so bad for life on Earth becomes quite problematic.
But the main argument against abiogenesis on Earth is simply the lack of evidence in favor of it; it shouldn't be a surprise that that things don't add up when one assumes the truth of a hypothesis that is not supported by any facts.