Shpongle

Could abiogenesis have happened more than once? And could it still be happening today?

Wounded King

This is obviously a highly speculative area but I cannot see why if abiognesis could have occurred in a certain set of primitive conditions it need only have occurred once, it may depend on exactly how abiogenesis actually occurred though.

As to abiogenesis going on at the moment, at least spontaneously, I think this is less likely. Certainly given the differences between our modern biotic environment and any theoretical prebiotic environment, the neccessary factors seem to be unlikely to still be present. This only matters if the original abiogenetic event relied on a specific set of conditions particular to the prebiotic environment. If life originated near hydrothermal vents it may be that the conditions still exist for abiogenesis to occur. Im not sure how much that particular environment has changed in the intervening periods of geologic time.

Doubting Didymus

New abiogenesis replicators would have to compete with the already present and much more finely adapted organisms, which fill just about every available niche these days.

This question is difficult to answer with any kind of specificity because we don't yet know for sure what the abiogenesis thing was, we only have good leads. Once we have a better understanding of what happened where then we can assign some probability to its occuring many times.

Coragyps

Any modern contenders for starting new life, other than maybe one in a particularly long-lived mid-ocean vent, would also have all this pesky oxygen to deal with. Even the deep ocean has a pretty good dose of O2, and it isn't trivial to deal with until you've developed at least a cell membrane and some peroxidase sorts of enzymes.
All the hungry bacteria we have everywhere around us would probably be an even bigger barrier.

Arthwollipot

Pretty much all of the evidence I have seen suggests that abiogenesis happened only once, and it is not going on today.

For example, all living organisms use the same code to translate the string of As, Ts, Cs and Gs into chains of amino acids. There is no reason to think that if there were independent abiogenesis events they would all have come up with the same code.

Any secondary origin of life would have to compete with the first (us). We have billions of years of evolution behind us, and are better at survival then they would be.

Wounded King

The ancestors of a second abiogenetic event need not be around today, assuming that more than one had occurred. As DD pointed out they may simply have been outcompeted by the life already established, and this could have happened at a very early stage in their development.

If there were such secondary events it is unlikely that we will ever know about them.

Darwin's Beagle

Good questions! Abiogenesis COULD have happened any number of times. The unanswered question is how fast does it occur. It is thought that the earth's surface did not cool sufficiently to have a solid surface until 4 billion years ago. So that is thought to be the earliest point in time that life could possibly be supported. The first true microfossils are dated at 3.5 billion years ago. If that corresponds to the first time that living organisms arose then it would take about 1/2 billion years for abiogenesis to occur. In that case, it only happened once.

However, there is data suggesting that life arose significantly before that. Gustav Arrhenius and Stephen Mojcizs (sp?) looked at apatite inclusions in 3.85 billion year old rock from Akilia Island near Greenland. They found the Carbon-12/Carbon-13 ratios to be enriched. This suggests that there was an active life-like chemistry going on at that time and possibly earlier.

If this is true (and there is quite a bit of controversy as to whether or not it is), then abiogenesis could be quite quick (I have heard speculation on the order of decades and centuries). It is possible that life arose a number of times on early earth only to be destroyed by the relatively frequent bombardments of meteors during that period of time on earth.

In any case, it is thought by most scientists that all present life is derived from a single abiogenesis event.

For abiogenesis to still be going on is much more difficult. Any simple form of life would have to compete with the existent life on earth which is much more likely to be better adapted. Having said that though, Gunther Wachterhauser did speculate that abiogenesis events may still be taking place around iron pyrite deposits along deep sea vents.

Regards

Darwin's Beagle