Fish may have evolved to live on land more than 30 times

[Vertigo]

Single originating event and common descent from it is more likely than many independent ones.

I would go further than "more likely"; I mean what are the chances of totally independent origins resulting in common shared genes? It's just too unlikely to be possible. In my opinion if there were any other abiogenesis events then they were less successful and were out-competed by the one we know. However I think that also pretty unlikely as in the early days there was plenty of resources to go around (therefore less need for competition) and I'd have thought any other life would have at least reached fossil levels before being eliminated and there's no real evidence for that.

And yet they continue to look for a single ancestor for all life:
Universal ancestor of all life on Earth was only half alive

I think that's really fascinating, Brian. It makes absolute sense that there must have been a half way stage (and quarter way and eighth way....) in the origin of life, whereby there was something that shows many but not all of the features we would identify with life. Although we talk about abiogenesis as thought it was an instant (biblical even) sudden leap from inanimate to animate, it's far more logical for that to have happened gradually and it's fascinating that hydrothermal vents might have provided the initial crutch needed before life managed to evolve all the required processes.

 

[Dennis E. Taylor]

Single originating event and common descent from it is more likely than many independent ones.

It also could be that there were multiple originating events at around the same time, but all died out save one. There would be no record. However, if this was the case, I think it would mean that the particular environment was prone to generate a particular type of life, so I'm not sure we'd be able to tell them apart at this point anyway.

 

[Vertigo]

It also could be that there were multiple originating events at around the same time, but all died out save one. There would be no record. However, if this was the case, I think it would mean that the particular environment was prone to generate a particular type of life, so I'm not sure we'd be able to tell them apart at this point anyway.

I disagree, in as much as, yes, there may have been other independent events that subsequently died out but if they didn't die out the chances of them duplicating identical sets of anything as complex as genes is so unlikely as to be virtually impossible even if they were 'created' by the same evnironment. And they would therefore be easily identifiable as they wouldn't share the common gene sequences all life shares. Remember that DNA and genes in particular are essentially just sets of instructions and the chances of two completely different sets of instructions being complementary and sufficiently compatible to allow cross breeding is hugely unlikely.

 

[Brian G Turner]

I'm suspicious of humanity's predilection for trying to apply a simple answer for complex problems.

We've seen in the fish example that the potential for life to have moved from the sea to land could have happened multiple time - we've seen in another thread about how the idea of modern humans originating from a single ancestor is now regarded as ridiculously over-simplified.

If there's a single ancestor to all life on earth then it presumes that abiogenesis is effectively a miraculous occurrence - so improbable as to be against natural laws. However, if there is an ordinary physical process that, under certain conditions, makes such a process possible, then IMO there is no way all life began from a single miracle cell, but instead, that the process would have resulted in multiple occurrences, even if within the same immediate location.

And as has been seen, the swapping of genes is especially common among simple lifeforms - therefore IMO rather than trying to identify a single individual, it would seem more realistic to identify a single community. And that's presuming that abiogensis occurred only once on earth. IMO - as with hominids - we are eventually going to find that the answer is far more complex than that.

2c.

 

[Dave]

IMO there is no way all life began from a single miracle cell, but instead, that the process would have resulted in multiple occurrences, even if within the same immediate location.

And as has been seen, the swapping of genes is especially common among simple lifeforms - therefore IMO rather than trying to identify a single individual, it would seem more realistic to identify a single community.

I would also point out mitochondria as clearly being examples of some kind of small proto-cell that were enclosed and engulfed by some larger proto-cell. I suggest that something very similar must have happened many different times before this actually "worked." Then you have clumps of identical single cells differentiating and having different properties but working together as a colony before ultimately evolving into a single organism. It seems very unlikely to me that that would have worked on the first occasion, but it obviously happened more than once in different ways on several different occasions to produce different kinds of differentiated cells.

IMO this kind of repetition over great lengths of time makes what Brian describes even more likely.

 

[Vertigo]

Well I'm absolutely no expert in this field (or any except software really!) but I just struggle to image how such swapping of genes could happen between anything that is not already related. It seems to me that even in the simple 'half life,' described in another recent thread, that half life is still chemically extremely complicated and I just fail to imagine a situation where another equally complex chemical 'life form' would even have compatible genes to swap. The assumption being made here, and I consider it to be a huge assumption, is that two separate completely independent creation events would present such an incredible coincidence as to be using a gene mechanism so similar that they could swap those genes. It just seems to me to be so far outside the realms of probability and far more unlikely than the idea of one successful creation (possibly/probably after many 'failed' attempts as Dave suggests) which then goes on to duplicate itself. Once it has achieved that it is almost inevitable that, with nothing except catastrophe to stop it, it will eventually colonise every possible niche on the planet.

I think it comes down to a number of facts for me - though I'm not sure if they are all 'facts':

Life is an incredibly complicated chemical process.
All life on Earth shares a lot of genes suggesting strongly that it is all related.
Only extremely closely related DNA can naturally combine (cross breed). Any significant difference and you have a different species and natural cross-breeding simply will not happen (without us doing the genetic intervention/meddling).
Even when closely related, such as chimpanzees who share 98% of our genes, we still can't cross breed with them

So the idea that a completely different life form would have the same sort of genes that would allow combination seems highly unlikely.

I would also add that the assumption that such a thing could happen naturally extrapolates to the assumption that all life in the universe (assuming there is other life) will be equally similar. If not then why is there no other completely different life to be found here on such a life friendly planet as Earth? In other words if we are saying that there have been many different abiogenesis events on Earth and they have all somehow been sufficiently compatible to merge then the same should be true across the universe, at least on any planet remotely similar to the Earth.

 

[hopewrites]

My understanding is that the more complex organisms have harder times crossbreading than simpler organisms.

If DNA is instrustions, think life as things to be built. Simple organizems would be like bicycles or skate boards. Easy to interchange parts -cross breed. But the more complex you get, motor bikes cars, planes, space stations... the harder it is to find another manufacturer who has compatible parts.
(Yes I know this is an imperfect analogy because manufacturers will purposely make their parts NOT be interchangeable where possible so you have to buy their parts, often from their mechanics...)

New or Pre life wriggling around in what I've always heard called "primordial soup" swapping bike tires and chains, trying out new and better seats handle bars, adding and dropping features like bells baskets or umbrellas, expanding to 2-12 person bikes contracting back down... Seems eminently plausible to me.

Once bio-technology reaches the stage we humans are at, yeah it's going to be hard to find something, even by the same manufacturer, with enough interchangeable parts for interbreeding.

 

[Stephen Palmer]

I must once again point to this exceptional book, which gives answers to the above questions, and more. Buy! you won't regret it…

 

[Brian G Turner]

I'm absolutely no expert in this field

Neither am I - everything I say is personal conjecture.

I just struggle to image how such swapping of genes could happen between anything that is not already related

It's still relatively new to science - ie, not properly understood - but some living organisms can, under certain conditions, absorb genetic material from their environment into their own DNA:

Horizontal gene transfer - Wikipedia, the free encyclopedia

 

[Vertigo]

Yes but that is genes. Why should an independently created life form even have genes or at least chemically compatible genes. The assumption being made here is that DNA with genes organised exactly as our are is the only way life can happen. And that may well be right (after all we've yet to find any evidence for any other mechanisms despite how many chrons like to speculate on life that can thrive in a high radiation environment which DNA certainly can't) but even if there was an completely independent DNA based life form I still can't image that it's 'genes' would be compatible with ours. It just seems so unlikely.

The way I look at it genes are like biological bits of software and this all seems about as likely as the virus in Independence Day being able to infect a completely alien operating system. Sure they might both use binary code - but that won't make them compatible.

And yes @Stephen Palmer it is on my list and I will get around to it... sometime!

 

[Brian G Turner]

Why should an independently created life form even have genes or at least chemically compatible genes. The assumption being made here is that DNA with genes organised exactly as our are is the only way life can happen.

Oh, totally agree - almost certainly there are other possible variations.

What I'm suggesting is that if you have a specific set of conditions that can give rise to abiogenesis, then isn't it possible that other structures - following the same pattern - could also have developed in the immediate locality?

And if similar conditions and localities exist in a general area, then is it possible for similar lifeforms - perhaps some with key differences - to also independently develop?

The example given for a single ancestor is that of forming at a hydrothermal vent, but in the modern world different systems of hydrothermal vents have their own unique yet similar species.

And here's something to complicate things - there has long been an argument within science that life arrived from space on meteorites or within comets. We know bacteria can survive in space - heck, even Water Bears can. So...even if life began on earth, what if additional building blocks for life arrived from space that allowed for completely new genes to be assimilated and/or developed - thus giving evolution a big push?

 

[J Riff]

We must ask the ancient Spider-gods. They probably were flying around millenia before any birds, watching it all unfold. That's my theory. *

 

[Stephen Palmer]

Horizontal gene transfer is for bacteria & archaea.

 

[Vertigo]

Oh, totally agree - almost certainly there are other possible variations.

What I'm suggesting is that if you have a specific set of conditions that can give rise to abiogenesis, then isn't it possible that other structures - following the same pattern - could also have developed in the immediate locality?

And if similar conditions and localities exist in a general area, then is it possible for similar lifeforms - perhaps some with key differences - to also independently develop?

The example given for a single ancestor is that of forming at a hydrothermal vent, but in the modern world different systems of hydrothermal vents have their own unique yet similar species.

And here's something to complicate things - there has long been an argument within science that life arrived from space on meteorites or within comets. We know bacteria can survive in space - heck, even Water Bears can. So...even if life began on earth, what if additional building blocks for life arrived from space that allowed for completely new genes to be assimilated and/or developed - thus giving evolution a big push?

I think your first point might be possible but only at the most basic and very earliest stages of life; the higher the complexity the less likely that sort of mechanism is, but even at the most basic level, for me, the complexity already present before you get even the most basic sort of half alive suggests a single event far more likely than the merging of multiple events.

Yes there are different species at different vents but today those species share DNA (along with all other life) so I still feel colonisation to be more likely than separate creation.

I'm afraid I've always been sceptical of the whole panspermia theory primarily because it just pushes the problem back further. If life came from comets etc. then where did that life originate? It's highly unlikely to have originated on the comets themselves. Just because they have organic compounds does not make them suitable for abiogenesis, I would think vacuum would still be a highly hostile environment for the creation of life. Ultimately the idea that an environment that is fundamentally friendly to life (liquid water etc.) would fail to produce life itself but instead be colonised from space - an environment far far more hostile to life - just seems immensely improbable and, for me, fails the Occams razor test.

Horizontal gene transfer is for bacteria & archaea.

But even then they already share the same DNA mechanism; they are already related.

 

[Brian G Turner]

Just adding a couple of notes to this thread from Astrobiology magazine:

Evolution Influenced By Temporary Microbes - Astrobiology Magazine

volutionary biologists often think about evolution as working on the level of organisms, with the individuals that are best adapted to their environments most likely surviving to pass on their adaptations to their descendants and perhaps to their species as a whole. A recent tweak to this concept is the so-called hologenome theory of evolution, which considers evolution working on the level of the holobiont.

However, some microbes do not fit well inside the hologenome theory of evolution for a number of reasons.

New fossil evidence supports theory that first mass extinction engineered by early animals - Astrobiology Magazine

Newly discovered fossil evidence from Namibia strengthens the proposition that the world’s first mass extinction was caused by “ecosystem engineers” – newly evolved biological organisms that altered the environment so radically it drove older species to extinction.

 

[Dave]

I do see your points Vertigo and you may well be right. Certainly, no one should take what I have said as "truth" as it was just speculation.

I'm afraid I've always been sceptical of the whole panspermia theory primarily because it just pushes the problem back further. If life came from comets etc. then where did that life originate? It's highly unlikely to have originated on the comets themselves.

You are correct in this, it only means that the life came from elsewhere - a planet that no longer exists, a God or a "godlike" being (the preservers in 'Star Trek') or the @J Riff spider-gods. Therefore, it explains nothing about the origin of life unless it was a miraculous event.

However, if the Urey-Miller experiment option is true, and it happened within different solutions of organic molecules in different pools of water drying on the shores of ancient Earth oceans, then I see no reason why conditions could not be such that the same event happened repeatedly. In fact, if it was possible chemically once, I think it must have been possibly frequently.

For me, it comes down to a question of whether it was a miraculous one-off event, or if it is just a natural extension of universal physical and chemical constant values that were set at the time of the big bang. If the latter is true, then it stands to reason that it must be repeatable.

 

[Vertigo]

I do agree with you @Dave, that it might have happened several times in the way you describe. But the chances of each separate event producing something as complex as genes that could actually be compatible with each other seems remote. So if it happened multiple times I suspect it's most likely that only one 'variety' survived in the end. Whether that's survived the environment or survived competition is probably equally likely. But I'm convinced we all come from a single event. In fact Nick Lane argues that there was only one single event that separates simple stuff like bacteria from complex life based on the same arguments. So maybe we're looking at the wrong point. Maybe there were many events that created simple bacteria and archaea but only one single event where complex life formed. I don't suppose we ever will know the full answer.

 

[Brian G Turner]

And apparently some species of fish are still evolving to move on land:
These fish are evolving right now to become land-dwellers

 

[BAYLOR]

And apparently some species of fish are still evolving to move on land:
These fish are evolving right now to become land-dwellers

I for one welcome our new Fishy Overlords.