I'm doing some thinking on the prevalence of life in the universe. Does anybody have any info they can give me regarding the following question?

How many times has life begun on earth?

Or, if you lean towards such a theory, how many times has life arrived on earth from somewhere else?



  1. Well, how are you defining “life” here, Jonathan?

    Humanoid life?

    Bacterial life?

    Vegetable/green life?

    More info, please…

  2. so far every version of life that we have seen has the same mechanism for encoding proteins. the supposition is that all life that we have seen is therefore descended from the same source. this would make the answer a tentative “once.” But like everything in science, there is an asterisk next to the answer that notes “until we find contrary evidence.”

  3. Dennis, you say the same mechanism for encoding proteins. Forgive my ignorance, but am I right in saying this is different to having the same D.N.A. (in the same way that we can trace our human D.N.A. back to a particular group of humans in Africa. If I am right, why would having the same mechanism mean that there was most probably only one beginning to life on earth?

  4. I agree with Dennis. Basic life seems to have appeared but once. However, extinction level events which have wiped out whole swathes of macroscopic life have occurred a number of times according to the geologic record. But there is no evidence in that record that it sterilized the planet of all life. The microscopic level always survived in many forms.

    So Mother Nature was given a clean slate a number of times on the larger scale and evolution began again. The last such event likely being the asteroid or comet that created the Chixulub crater 65 million years ago in Yucatán and triggered an extinction event that wiped out the dinosaurs, thus allowing the rise of the mammals and ultimately our ancestors.

  5. thus allowing the rise of the mammals and ultimately our ancestors.

    So, we have a bloody asteroid to blame for Mexicans. Where’s Bruce Willis when you need him?!!!

  6. I did find some moldy cheese in my fridge just recently. Is that what you’re going on about?

  7. All living organisms on earth have nucleic acid but can express as DNA with assistance of RNA or just as RNA. It is the universal use of the four nucleobases (A, G, C and T) (U instead of T in RNA) to encode genetic material.

    There is nothing requiring that life chose this particular way to encode amino acids. But on this planet, from single cell bacteria that hover around ocean thermal vents to every other form of life, this one complex system is the only method that we find. Life could have found other methods that do the same job. Plenty of other methods have been postulated and suggested as ways to do it. But only one way exists in this world. Hence, it is supposed that there is only one line of descent from one starting point. The odds against the same complex method happening twice in the same place (when other ways could also work) are pretty astronomical. Go with the simplest explanation: one starting point for every life on this planet. (Occam’s razor and all that.)

    The one fly in the ointment would be prions. Are folded proteins that can replicate themselves “alive”? Or just a chemical compound with the ability to generate copies and cause bizarre diseases like Creutzfeldt–Jakobs and “mad cow” disease.

  8. Thanks, Dennis. Do we know that all living things on Earth at present are descended from an original single life form? Or is there a possibility that very basic life forms have come into existence in separate events?

  9. “is there a possibility that very basic life forms have come into existence in separate events?”

    Well, that is what comparing the mechanism of protein encoding seems to show: one common point of origin. Multiple points of origin would result in multiple methods of encoding genotype. But that hasn’t happened. At this most basic level every organism in the world has basically the same method of encoding the genotype. If there had been multiple starts it is practically impossible that there wouldn’t have been other methods for doing this.

    Multiple events would mean little to no chance that a very complicated system for encoding would have been used by every single organism.

    It was a one-off event.

  10. When a beautiful woman and I make love, then can it be said, for me, “Life has begun!” 😉

  11. Dennis
    that leaves me as a non scientist with the question of whether there might have been other attempts but evolution allowed only one system to flourish on earth.

    And whether there could be completely different systems in other parts of the universe.

  12. Erika, if it happened, we have not found the record of it having happened. Out of sight, out of mind. It could have. It might not have.

    There are many cosmologists that do not believe that there is any other life in the universe. And those that believe that if there is, no more advanced than we are. Certainly not advanced enough to be visiting Earth in space ships.

  13. This is why I asked the question, Dah-veed.

    Previously, because of the uniqueness of the earth (especially in respect of its relationship with the moon) I have tended to believe that sophisticated life will be extremely rare in the universe, but that primitive life may be common. However, if there was only one instance of life beginning on earth than I have to revise my view to believing that any form of life will be extremely rare in the universe. In fact, without a creator (and I don’t necessarily mean a sentient creator) I don’t think life should exist at all, let alone, advanced life like animals.

  14. Life forms other than the ones we have now may have arisen either before or after our strain, but we have no evidence for that.

    There are teases like the early multicellularity recently reported in Nature, but even though those organisms have no living descendants that doesn’t mean they were different from the form of life today.

    If, MP, you view these organisms as different “very basic life forms”, then you might argue that there has been more than one form of life on earth. But there is no evidence that there has been anything other than the current DNA-RNA scheme; if there was, it has not survived.

    None of which addresses the rarity (or not) of life throughout the universe. As “unique” as Earth may be, planets orbiting other stars are not uncommon. And as they say, if you’re “one in a million”, there are 1,324 people just exactly like you in China. Given the 100-400 billion stars in our galaxy, and the 80 billion or so galaxies in the observable universe, and who knows how many unobservable ones, Earth’s “uniqueness” may well be fairly common.

  15. In fact, without a creator (and I don’t necessarily mean a sentient creator) I don’t think life should exist at all, let alone, advanced life like animals.

    I agree. Miracles still happen. 🙂

  16. The bottom line is simply ‘we don’t know’. As a teacher of astronomy, we have only frequency of light to guide our research. We have a universe that stretches 13.7 billion light years in radius (that we can see), and we have no way of knowing if God creates lots of life, and isolates it by huge differences, if God gives galaxies to the Mormons, as suggested by their theology, or if life is rare in the universe because of or because of the lack of, a Creator.

    But, Jonathon, we do know that as we go from one stage to another in evolution, there are some amazingly convenient happenstances that must have happened to move from one level to another in evolution. Guided evolution, a very rich tapestry of coincidence…who knows? We just see the end result. And, for all its pettiness and spite, it is Glorious.

  17. Earth’s “uniqueness” may well be fairly common.

    That’s garbage, Paul (A). You have swallowed a ridiculous lie put out by NASA so that they can get more funding. But I doubt if there are many people in NASA who know so little about probabilities and the factors involved in maintaing life on earth (let alone starting life off on earth) that they believe it for a nano-second. The only reasonable possibility of life being common throughout the universe is if it evolved very early on in time, when the stars were closer, and somehow got distributed via inter galaxy travel. But it is generally accepted that nothing would survive the impacts.

  18. Of course, I have to correct you, Mimi. It wouldn’t be a miracle, merely a scientific fact. The word “miracle” actually doesn’t mean anything. God intervening is no less scientific than you stopping a child from running into the path of a car. If you were in the right position you could even observe God doing it.

  19. Yes, of course, MPA. If God is the creator then there can be life everywhere and anywhere (subject to whatever constricts God’s creative process, e.g. does God have to work to the rules inherent in the universe, created or otherwise). Personally, I think the chances of life starting in the universe, including on earth, is zero and that evolution to the life forms we have today is impossible, without an “intelligent” force being involved. But, if everything is accidental then I cannot see how life could be common given the mathematics of it all and most of my readers have a much higher view of the value of science, in respect of such things, than I do. the irony is that is a lifetime interest in science and all the facts I have learnt through that which has led me to a belief in intelligent design (although not the bog standard creationist one).