It has been seriously proposed that the Ediacaran biota MIGHT represent a “first draft” of life that failed and was replaced by the “modern” forms in the Cambrian Explosion.
The whole Ediacaran fossil record is ambiguous and poorly understood.
But as a “first draft” of multicellular life, not of life in general. They are at least usually assumed to be eucaryotes, though perhaps not classifiable with extant animals, plants, or fungi. (A few forms have been proposed as representing bacterial colonies.) I don’t think anyone has proposed that they were of entirely separate origin from modern organisms.
While these different forms may be theoretically possible, or actually possible to some degree, it’s also possible that they aren’t viable in the long run.
Here is an on-line paper (pdf, technical) which goes into much detail about the chemical basis for life. For example, it mentions some of the benefits of the phosphate chain in nucleic acid. (I’m suspicious of the claim that there a million alternatives to nucleic acid! Do the alternatives also use phosphate chains?)
Minor point: Some organisms (or mitochondria) have an extra amino acid or have deviations to the standard genetic code.
Re-reading this, I’m sure it will be misinterpreted. I am NOT saying that a phosphate chain is an absolute necessity for any genetic coding molecule. I AM saying that nucleic acid, and other hypothetical coding molecules, may possess serendipities, some of which we’re unaware of.
Beat me to it. He named his book about the Burgess Shales Wonderful Life because the same principle underlies the Capra film with a similar name: small, often random occurrences at key moments in history can have huge effects, locking in one template (but none of the others) for all that follows. This is about what happened to multicellular life forms half a billion years ago, not the origins of life itself discussed in most of this thread, but the principle is the same.
Cool links. One of the critters mentioned is bigger than a bacterium or protist: the silk moth Bombyx mori.
(Never mind. The silk moths were engineered, not a natural occurrence. Still, interesting that they were perfectly viable.)
Assuming all life on Earth arose from a single glob of chemicals at a particular time somewhere in Earth’s primordial soup, I find it humbling to consider how vulnerable our present biosphere must have been at Time Zero, due entirely to chance.
Let’s consider the tide causing a pebble to bump into the initial glob, destroying it.
What would have happened next? What are the possibilities?
Maybe the precursor to life would never form again and Earth to this day remains sterile. I find that scenario unlikely, but who knows.
Maybe a similar glob of chemicals would combine shortly after the first one was destroyed and life would kick-start and progress essentially the same as it did. But, how similar would it be?
On the one hand, all of Earth’s inorganic events would remain exactly the same (e.g. tidal forces, tectonic shifts, bolide impacts, etc.), so the major stressors directing evolution would be the same. Would starting with the same chemical glob, but time-shifted hours, or centuries, or millennia from initial formation, acted upon by the same evolutionary stressors, lead to a near exact biosphere billions of years later? You wouldn’t be here, but maybe your doppelganger would be. Maybe he’d have a tail, reproduce by budding and enjoy country-western music, but otherwise be human-ish.
On the other hand (or tentacle), perhaps something else, like quantum effects would dominate and cause life to evolve in a very *dissimilar *fashion? Perhaps intelligence would still be selected for over time, but the dominant species on Earth would be squid people?
What if, after our bio-line’s initial chemical glob was destroyed, a glob of *different *chemicals evolved organically to create our biosphere? I doubt the resultant lifeforms would look much like today’s lifeforms at all, but perhaps the basic forms, functions and stratification would remain essentially the same due to Earth’s non-changing physical events and processes directing its evolution. In other words, would conditions on Earth form similar biospheres no matter what organic glob started the ball rolling?
No way to make much more than educated guesses, of course. But, thoughts?
Well, per chaos theory, small differences propagate over time.
I think it’s likely that at the time that we had a glob of proto living goo first become a true living thing, we probably had millions of these globs all over. So – if life formed in hydrothermal vents, or in tidal pools – many tidal pools or vents around the works likely had proto living goo.
But if the goo that we all come from was destroyed, and different goo evolved into the first cells… who knows? I think life on Earth would be entirely unrecognizable. Some things might go the same way-- like, we would probably still see the rise of bilateral symmetry, because that’s a very efficient design that animal life stuck to as soon as it evolved. But in the specific details? Life would be as different as it would be on an alien planet, despite similar selection pressures.
I can’t access the original article, and it’s difficult to assess exactly how they did their calculations. I’m not sure that they excluded phosphate chains as one way to link the bases. However, they include stereoisomers so that this accounts for an additional order of magnitude in the number of variants.
Saw an article a little while ago about how cell wall membranes could develop a lot more easily than was expected to create the first proto-cells; that is, collections of reproducing molecules enveloped in a protective bubble. Their current speculation was that it was deep sea volcanic vents that provided the material, energy and environment for life, rather than tidal pools. These vents would have been a good source of the minerals (including phosphate) that helped drive these reactions.
But think of life as like Betamax and VHS, or some such; if two competing niches evolved separately, sooner or later one will be an invasive specie in the other’s environment, and utterly dominate the environment. We see much higher level versions of this separated evolution like the lemurs of Madagascar, the finches of the Galapagos, or the hopping marsupials of Australia - but the world’s oceans were interconnected so any life forms evolving in there would spread across the globe fairly easily, and the less competitive life forms would become lunch. Whichever life process is best adapted to evolutionary mutation probably wins the race - absent Gould’s random chance factors.
Are there any thoughts of what the DNA of that common ancestor might have been? Since it happened so long ago, I would think that the common ancestor would have evolved in many different ways and the resulting genes we see in organisms today would vary widely. Did that common ancestor have most of the same genetic code we see in organisms today? For example, when animals and plants share the same genes, does that imply that the common ancestor also had those genes?
I agree with your post, except for this part:
<snip>Life would be as different as it would be on an alien planet, despite similar selection pressures.<snip>
If you innoculate primordial Earth and a large sample of life-friendly exo-planets with a bolus of organic goo, I believe we would, with a fairly high degree of accuracy, be able to tell what resultant biosphere evolved on Earth, due specifically to the unique selection pressures of each planet.
For example, I believe planets with few physical selection pressures would tend to evolve rather dull, dumb lifeforms. Sure they’d have to evolve ways to out-compete other lifeforms, but they wouldn’t have to figure out how to thrive in their goldilocks environment. Maybe that planet doesn’t need to evolve a large variety of lifeforms to fit a large variety of niches, because it doesn’t *have *a large variety of niches. Maybe they only evolve a small number of species and they all get along with each other, in kumbaya fashion.
Alternately, planets with selection pressures much greater than Earth would, IMHO, tend to evolve lifeforms much craftier and more resilient than Earth’s lifeforms. You don’t want to piss those guys off, they’ll eat you for lunch!
Perhaps radial symmetry would dominate over bilateral symmetry on water worlds.
Earth’s pressures created lifeforms suited to Earth’s pressures, and I believe we could identify those in a random sampling. As one (of many) examples, our K-T Mass Extinction occurred when the dominant species on Earth were getting too big for their britches. The Chicxulub asteroid knocked those lumbering, entitled dinosaurs on their collective asses and allowed sneaky rats to fill in the void. Classic David and Goliath scenerio. Brains over brawn. Ali vs. Frasier.
That’s a very outdated view of dinosaurs. They weren’t lumbering nor entitled, and they survived for far, far longer than any of the modern mammalian lineages.
For context, tyrannosaurus rex was around for 2 million years and (with similar species in the genus) had a huge distribution across North America and Asia. That’s an impressive spread across time and space for a large carnivore; we just barely split off from chimps 2 million years ago.
Or look at the group of Sauropods as a whole. They were THE dominant land herbivores for a mind-boggling period of time - over 100 million years. Mammals as a whole have dominated land for only a fraction of that time, and none as successfully as the sauropods.
Dinosaurs were the dominant landform for nearly half of the time when vertebrates lived on land, and they’re still around today in the rather successful avian form. They were certainly not lumbering buffoons asking for extinction.
Wikipedia has thoughts! 
Here’s a paper:
Note that (a) genes common to all life, (b) a minimum genome, (c) a minimum self-sufficient genome — form an ascending-size series. (The human genome is very rich, but not self-sufficient: we depend on other life for some essential vitamins and amino acids.)
Interesting idea, but it’s actually quite difficult to tease apart organic from inorganic at a geologic scale. Many geologic processes that might seem inorganic at first glance are highly organic in fact.
For example, the water cycle is massively changed by living organisms. Life slows down the flow of precipitation from the atmosphere to the oceans. Transpiration sends water back into the atmosphere sooner, increasing precipitation over the land. Precipitation impacts river and glacial flows. This affects the contours of the land and the formation of sedimentary rocks. And of course sedimentary rock (and the metamorphic rock produced from it) is found in many places.
More obviously, the oxygen and carbon cycles are heavily mediated by life and impact geologic processes.
If life never happened on earth, the planet would look vastly different than superficially removing only life.
The basic genes that govern fundamental metabolic processes cycle are highly conserved, and are found from bacteria to multicellular organisms.
From here
Escherichia coli is a bacterium. The fact that the enzymes are similar implies that the genes producing them are also homologous, although they may be regulated in different ways in bacteria and eucaryotes.
I’m not sure what you are referring to by “genetic code,” but the DNA base codons that correspond to the individual amino acids in the proteins produced by the code are (with minor variations) the same in all life on Earth today, which is the major evidence that it has a common origin.
Yes, but note that the common ancestor of all life we are talking about was probably more than two billion years earlier than the common ancestor of plants and animals.
Placing plants and animals on to the Tree of Life is complicated by the fact that plants have plastids, with their own DNA, inherited via an ancient symbiosis with a cyanobacterium. And (almost) all higher life have mitochondria, with their own DNA, inherited from an even more ancient symbiosis.
In addition, the role of “horizontal gene transmission,” or transfer of genes between disparate lineages, is being increasingly apparent. Many genes in humans and other animals are from bacteria and viruses.