ONE common ancestor??

[Manny]

What I was referring to though, is that there are several thousand potential ways to code the replication sequence all life on Earth has, in DNA with our 20 amino acids, but only one of those is expressed, suggesting a single ancestor.

The current theory is that there was a pre-RNA stage, sometimes called "PNA World", which became the "RNA World", possibly (but not yet certainly) due to superior efficiency, and then DNA evolved and we have the above.

John's comment also applies. It is known that life could not have evolved in our current environment. What did evolve was what was probably best-suited to the environment at the time, and once Earth became sufficiently oxigenated, it was no longer possible for another rival to show up.

Cool, that's helpful, thanks. Is there something like a "Current Hypotheses of Abiogenesis for Dummies" kind of thing?

 

[Manny]

IIRC, there's a fossil bed in Canada that had all sorts of strange creatures in it. They think the creatures co-existed with the trilobites, but were far superior in design, at least as far was we can tell. They existed in a lake, actually, that wasn't connected to the regular oceans.

Are you thinking of the Burgess Shale? There were several phyla there which apparently didn't survive far into the Cambrain, but I don't think there's any indication that they arose from a different common ancestor to other life.

 

[chipmunk stew]

My own thinking is that DNA wasn't an issue at the point that life began, but came about afterwards. There are other, less-stable self-replicating molecules, and ones that do not take large, chance-designed enzymes, to decode and recode, as well.

The fact that not all species and organisms decode DNA quite the same would argue that there is some diversity, but I think what argues more is the kind of diversity shown by things like archeobacteria, some kinds of fungi, and a variety of niche organisms.

It's also possible that there were some self-replicating proteins that actually were more stable (better at copying themselves) than RNA/DNA. Perhaps this even gave them a head-start at first. But then, perhaps, the weakness of the RNA/DNA molecules (relatively poor self-replication) emerged as a strength in that its rapidly expanding, mutation-driven diversity allowed it to spread and adapt to a variety of environments and competitive situations, triggering evolution.

 

[clarsct]

AHHHHH!

Thanks. I think that was it. I was saying that they had a different common ancestor to the trilobites, but that they were a POSSIBLE different ancestor to ourselves. A 'different puddle' so to speak.

But then the lake dried up....


There might've been many puddles along the way that dead ended for reasons we can't imagine. I guess that's my point. There's no way to really know, but there IS evidence that perhaps three puddles were joined along the way. Quite possibly, there were more.

 

[PatKelley]

I can eat a plant, but whether I can digest it is another matter. There might have been several types as the nutrients were sopped up, but one can only predate on creatures that have at least a similar makeup at first. RNA would have to rely on its ribozyme configuration matching nutrients in the prey that could be broken down, assuming it was something other than RNA. There might have been several RNA starts, several other types of starts, but eventually one won out.

 

[Xeriar]

I agree. The first was probably RNA, which can self-replicate more-or-less by itself.

RNA came before DNA, this is known.

Before RNA, it is believed that there was a "Pre-RNA world" soup, where many competing base combinations battled it out in the Arena that was early Earth, and RNA won.

 

[Xeriar]

Cool, that's helpful, thanks. Is there something like a "Current Hypotheses of Abiogenesis for Dummies" kind of thing?

To put it in a nutshell off the top of my head

1: Various base organic compounds form on the surface of the Earth and Mars, before it had any noticible oxygen. Certain compounds, and variations of these compounds, dominate from the get go due to better stability. Also note that some of these compounds possibly originated from cometary matter.
2: Some of these compounds form more complex structures, such as lipid bubbles, protein chains, base chains, etc.
3: On either Earth, Mars, or both, some of these chains developed the ability to completely self-replicate in varying situations. This is sometimes called the "Pre-RNA World hypothesis" or "PNA World". Currently, it's the most hypothetical part of this.
4: Eventually, one of these chains formed RNA. Either it got sent to Earth from Mars, or developed on Earth independantly. For one reason or another, RNA overtakes all other biological attempts on the planet, forming the "RNA World" hypothesis, though to my knowledge this is supported by chemical analysis of geological strata.
5: Eventually, a piece of RNA lost an oxygen atom and became DNA. It may have taken a few tries before the result actually worked, but once it did, it overtook the planet.

I'm sure people with more biology experience than me would be dissapointed in my explanation - as I'm aware, the actual process involves hundreds of steps, some of which can occur without others and can occur simultaniously but are still necessary for -our- form of life.