IS COMMON DESCENT ABSOLUTE

Why are scientist so certain that life evolved only once?
Is there no possibility that one line of cells gave us things such as horses, dolphins and Julia Roberts, and another line gave us lobsters, cockroaches and Eric Roberts?

Thread moved here from the Proposed New Topics forum.

No one is certain that life didn't evolve multiple times.However, there is good reason to say that all life today only comes from one line. The others seem to have died out or we haven't found them yet.All the things you listed and more all the way down to bacteria and archea have some very fundamental things in common. Down deep the basic cellular processes are all the same. The kind of things that you would expect to be conserved since any mutation in them is almost certainly going to be fatal to the organism.

You've asked two questions.It's very likely that life evolved a number of times, in several separate events.All life that exists today, that we know about, is the decendant of only one of those living things. All the rest of them died off. Since it's very obvious that all those things are related to each other, well, it might be possible, but not very likely. All living things we're aware of appear to be decended from the same original cell line; there may have been other lines at one time but they don't appear to have survived.

quote:

---

Is there no possibility that one line of cells gave us things such as horses, dolphins and Julia Roberts, and another line gave us lobsters, cockroaches and Eric Roberts?

---

At that level, no. There is almost no doubt that all eukaryotes are descendants of a single eukaryotic anscestor species (cells with mitochondria/chloroplasts and nuclei). However, it is very possible that there are multiple lines of descent very early in bacterial evolution. Bacteria are quite good at sharing DNA between very different species. It is very possible that several lines of early bacteria merged or melded over time. This process is called horizontal gene transfer, or HGT.

I've seen some mathmatical models that attempt to give an idea of the odds of life forming by chance from the type of enviorment we see the early earth was. The statistical chances begin to get enourmous as all fo the different factors are weighed in against living cells being formed at all. I cannot personally verify the accuracy of the numbers but the line of thought seems reasonable.What are the mathmatical odds of multiple seperate occurances of this existing where accident and chance result in a combination of chemicals that create life? So in the line we are familiar with you end up with a sort of machine code in the form of DNA which contains the instructions for building stuff.Maybe in another line of life form, the intellegence is incoded in a completely different manner other than code stored in DNA. What are the odds of that? For that matter, is there another way to preserve information in nature and replicate/evolve it to keep creating more complex organisms over time, other than the DNA information storage method?

quote:

---

I've seen some mathmatical models that attempt to give an idea of the odds of life forming by chance from the type of enviorment we see the early earth was.

---

And all of them are wrong for one reason, we don't know how life can arise through chemistry. Until we know all of the possible pathways through which life can arise through abiogenesis there is no way that we can assign a single probability. As an analogy, we can't know the odds of a lottery unless we know how many balls are in the hopper and how many balls we have to draw. For instance, if there are only 10 balls in the hopper then the chances of getting one ball right is one in ten. If we have to get 3 balls right then our odds increase. However, if there are ten balls and they draw out ten balls, then our chances are 1 in 1.And also, this is off topic so if you have additional questions it would be more appropriate in another thread. (no disrespect meant)

quote:

---

Maybe in another line of life form, the intellegence is incoded in a completely different manner other than code stored in DNA. What are the odds of that? For that matter, is there another way to preserve information in nature and replicate/evolve it to keep creating more complex organisms over time, other than the DNA information storage method?

---

Simply, we don't know. This is why it is impossible to assign odds to life forming through chemistry.As wild conjecture, life may have started separately as RNA and as protein. These two systems may have merged at some point to result in the life we see today. Or, hypercycles of amino acids within bubbles of phospholipids may have engulfed self replicating RNA. No matter what, we will never know how life started on earth but we may be able to come up with some possibilities.

Perhaps you would be kind enough to present one of the models here so the rest of us can examine and discuss about.

Thanks all for your responses.
If scientists came across a fossile of a single celled life form that had evolved seperately from the ones we came, would they have any method of knowing this?

quote:

---

If scientists came across a fossile of a single celled life form that had evolved seperately from the ones we came, would they have any method of knowing this?

---

It's very doubtful that they could. Bacteria are classified by their biochemistry (eg metabolism), how their cell walls are constructed, and the proteins that make up their flagella. These types of things are almost impossible to discern through fossils. The only way to truly know would be to look at their genetic makeup. Of course, DNA does not fossilize and can only be reliably preserved for up to 100,000 years.

Hopefully this isn't too far off topic, but one possible avenue in which we could find out the answer would be discovering life on another planetary body. If it were possible for us to analyze such life, it would give us tremendous insights into our own evolutionary pathways.

That's pretty much what I suspected the answer would be, but I was hoping there might a method of which I was unaware. Thanks for your input.

I believe you actually are on topic. Finding life on another planet would give us insight as to what to look for in organisms that reproduced by a method other than DNA replication.