My question is about panspermia but in reverse. It seems to me that humans will eventually attempt to colonize other bodies in the solar system. Time and tech permitting, I have little doubt we will alter our new surroundings for our survival and comfort. Is there any established theory that explores the possibility that planets evolve space bound life-forms as a sort of planetary "reproductive" process? Could humans be planet seeds?

There is a great deal of evidence that biological evolution operates on a "whatever works," level. There is no evidence that anything is affecting evolution in a direction that would increase the possibility of the appearance of life forms that can travel through space. Now some people believe that given the right conditions it is very likely that intelligent life will eventually evolve that could travel through space. For example they might think that if an asteroid hadn't hit the earth 65 million years then perhaps some dinosaur decendent would have developed high intelligence and advanced technology. But this is very different from suggesting that there is something pushing the process, such as a planetary desire to spread life to other worlds.

I'm curious about the common ancestor.

How is it determined that there was one common ancestor? Would the DNA of life on Earth look different if life had originated more than once? Would it look different if the common ancestor had originated on Mars? What about both Panspermia and life originating independently on Earth, would we see any evidence in the DNA or fossil record today?

I love the idea of Panspermia; we really would all be children of the universe. (My two little boys love to be told they are made of stardust!)

Well we know there was one common ancestor going back hundreds of millions of years because we share genes with sponges and other simple organisms. Prior to that, DNA could have developed more than once on earth, although just once would be enough. Also, the very first lifeforms on earth wouldn't have used DNA as it's kind of difficult to make and preserve. Presumabley they would have used even simpler molecules. Just what we don't know, although there are lots of good ideas. In a few years very simple living cells will be created from scratch in the laboratory and these will help give us insights into what very early life may have been like.

As for DNA being different, if we ran the earth over again from the start we wouldn't get the same DNA, it would be different even if the end result was much the same. This is how we will be able to detect truely alien DNA life. However, we don't know if DNA is the only molecule suited for complex biological life as we know it. I wouldn't be surprised if something else could work, but DNA is the only one we know will work at the moment.

So however life on Earth originated, whether by panspermia or not, however many times, one ancestor won out over all the others and evolved DNA and life as we know it?

Presumably once life has got started (for example on Earth) then nobody else gets a look in? I mean we don't see any 'new' life spontaneously originating on Earth today (only evolution). Do we know the mechanism behind this? Is it just that new life would not be able to compete; or have I got it wrong and it could still happen in some quiet corner of the planet?

Yes. Although the economical answer is it started once, but multiple starts are certainly possible.

Once life has started then the chemicals presumably required for new life to form become food for existing life and so new life would find it hard to get started. Also conditions now are much different from when life presumably started on earth. However, for all we know, new life could be forming right now without us being aware of it. We don't even know how simple existing earth life can be and still meet the definition of being free living life. There could be simple life which doesn't use DNA on earth right now that has always existed alongside DNA life and we haven't detected it yet.

Ronald,

Cool posts. Thanks.

Steve