ALL IN THE FAMILY: The common assumption that all life on Earth emerged from a single common ancestor has been a difficult hypothesis to prove. New analysis sorts out some of the competing theories.
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Earth's first life-form, floating in the proverbial froth of the primordial seas that eventually gave rise to trees, bees and humans, is not just a popular Darwinian conceit but also an essential biological premise that many researchers rely on as part of the foundation of their work.
In the 19th century, Charles Darwin went beyond others, who had proposed that there might be a common ancestor for all mammals or animals, and suggested that there was likely a common ancestor for all life on the planet—plant, animal and bacterial.
A new statistical analysis takes this assumption to the bench and finds that it not only holds water but indeed is overwhelmingly sound.
Was it not already obvious, from the discovery and deciphering of DNA, that all life forms are descended from a single common organism—or at least a basal species? No, says Douglas Theobald, an assistant professor of biochemistry of Brandeis University and author of the new study, detailed in the May 13 issue of Nature. (Scientific American is part of Nature Publishing Group.) In fact, he says, "When I went into it, I really didn't know what the answer would be."
Despite the difficulties of formally testing evolution—especially back across the eons to the emergence of life itself—Theobald was able to run rigorous statistical analyses on the amino acid sequences in 23 universally conserved proteins across the three major divisions of life (eukaryotes, bacteria and archaea). By plugging these sequences into various relational and evolutionary models, he found that a universal common ancestor is at least 10^2,860 more likely to have produced the modern-day protein sequence variances than even the next most probable scenario (involving multiple separate ancestors).*
"Evolution does well where it can be tested," says David Penny, a professor of theoretical biology at the Institute of Molecular BioSciences at Massey University in New Zealand and co-author of an accompanying editorial. Yet, he notes that evolution can make "testable predictions about the past (especially quantitative ones)" tricky at best. "That Theobald could devise a formal test," he says, "was excellent…. It will probably lead to a jump in what is expected of the formal evaluation of hypotheses, and that would help everybody."
Common ancestor acrimony
The mid-20th-century discoveries about the universality of DNA "really nailed it for people" in terms of establishing in popular—and academic—culture that there was a single universal common ancestor for all known life on Earth, Theobald says. And since then, "it's been widely assumed as true," he notes.
But in the past couple decades, new doubt has emerged in some circles. Microbiologists have gained a better understanding of genetic behavior of simple life forms, which can be much more amorphous than the typical, vertical transfer of genes from one generation to the next. The ability of microbes such as bacteria and viruses to exchange genes laterally among individuals—and even among species—changes some of the basic structural understanding of the map of evolution. With horizontal gene transfers, genetic signatures can move swiftly between branches, quickly turning a traditional tree into a tangled web. This dynamic "throws doubt on this tree of life model," Theobald says. And "once you throw doubt on that, it kind of throws doubt on common ancestry as well."
With the discovery of archaea as the third major domain of life—in addition to bacteria and eukaryotes—many microbiologists became more dubious of a single common ancestor across the board.
A test for evolution
Other researchers had put certain sections of life to the test, including a similar 1982 statistical analysis by Penny testing the relation of several vertebrate species. Theobald describes the paper as "cool, but the problem there is that they aren't testing universal ancestry." With advances in genetic analysis and statistical power, however, Theobald saw a way to create a more comprehensive test for all life.
In the course of his research, Theobald had been bumping against a common but "almost intractable evolutionary problem" in molecular biology. Many macromolecules, such as proteins, have similar three-dimensional structures but vastly different genetic sequences. The question that plagued him was: Were these similar structures examples of convergent evolution or evidence of common ancestry?
"All the classic evidence for common ancestry is qualitative and is based on shared similarities," Theobald says. He wanted to figure out whether focusing on those similarities was leading scientists astray.
Abandoned assumptions
Most people and even scientists operate under the premise that genetic similarities imply a common relation or ancestor. But as with similarities in physical appearance or structure, these assumptions "can be criticized," Theobald notes. Natural selection has provided numerous examples of convergent physical evolution, such as the prehensile tales of possums and spider monkeys or the long sticky insect-eating tongues of anteaters and armadillos. And with horizontal gene transfer on top of that, similar arguments could be made for genetic sequences.
"I really took a step back and tried to assume as little as possible in doing this analysis," Theobald says. He ran various statistical evolutionary models, including ones that took horizontal gene transfer into consideration and others that did not. And the models that accounted for horizontal gene transfer ended up providing the most statistical support for a universal common ancestor.
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22 Comments
Add CommentSo life just evolved once.... Or we ate all the other parallel forms. Seems unlikely. Maybe evolving life is a very very very rare event and we are here do to panspermia?
Reply | Report Abuse | Link to thisOr perhaps that "common ancestor" is the first logical life form to emerge given the elements making up the environment of this world.
Reply | Report Abuse | Link to thisSo no matter how many times life forms, it begins there.
Think about it.
Reply | Report Abuse | Link to thisWe might be totally alone in the universe! Or at least so enormously far from an other intelligent life form that it is impossible for us to ever detect them.
I think we are her because it is so exactly right conditions on earth, in the solar system and in this region of space. Then we had so enormously much luck when life started and now is the exact time when "we" the intelligent life form has started to look in to space and ask the grand question: Are we alone?
Answer: Yes, it is highly possible.
But the big question is how the subsequent life forms began. Where they made up of a different sequence of elements in our environmental world, or did it get rooted in the one that already formed?
Reply | Report Abuse | Link to thisLuck may have something to do with it, but luck isn't scientific (even if it is realistic). Statistically speaking, even though we are considered "lucky", the probabliity we are alone is small.
Reply | Report Abuse | Link to thisYou can account for all the "luck" you want, but when you define that luck, we can see that there are many (out of an infinite source) cases where it is the same.
Bottom line, even if we were extremely lucky, statistics say that the scenario is repeatable.
The odds might be so against us finding it, that we might not ever, but to say it is probable that it isn't there, is a huge leap.
Possible, yes, probable, no.
Also an interesting point is the possibility of life on Europa. There won't be any "intelligent" life there, but if we see life, it could be the common ancestor of the life on that planet. We cannot say for sure evolution happens on other planets, or at a rate that makes it noticeable, but if we were to find simple life-forms, this study can help show the possibility of intelligent life forming.
Reply | Report Abuse | Link to thisAn even more intersting point, if we did find simple life-forms, we could witness the early stages of life, where as we were not around to witness and document ours. That is where this study can become extremely important to more than just our past. Not to say that we'll be around billions of years to witness the whole thing!
If we go to europa, we will seed it with bacterial/archael/fungal life from earth.
Reply | Report Abuse | Link to thisIt is really hard to assign a probability of the appearance of 'life' when 1) you have a known statistical population of 1 (earth), and 2) you don't have a known mechanism for the initiation of life (Genesis doesn't count). You can throw all sorts of multi-magnitude assumptions together and make wild guesses, but in the end... we just don't know how easy/hard/often life starts.
Reply | Report Abuse | Link to this"So life just evolved once.... Or we ate all the other parallel forms. Seems unlikely. Maybe evolving life is a very very very rare event and we are here do to panspermia?" - Blah
Reply | Report Abuse | Link to thisA large forest fire is lit only once. Once burned it prohibits ignition from other sources. The probability of ignition at any point is vanishingly small.
I fail to see the problem.
"Also an interesting point is the possibility of life on Europa" - yawn
Reply | Report Abuse | Link to thisCold. Low energy. Slow chemistry.
It does make panspermia seem a bit unlikely, doesn't it?
Reply | Report Abuse | Link to thisWell here is the crux. What are his evolution models based on and what variables do they take into account? If his models follow the same exacting style of criteria that climate models do then we can safely throw this report on the same junkscience pile with most of the rest of what SciAm prints
Reply | Report Abuse | Link to thisI am always thinking that convergent evolotion implies there is a determinism in how life proceeds and what it caan produce as features. i can kind of extend this to the first life. that when life appeared there is only one possible combination of elements and characteristics. that there maybe only one possible primordial cell.
Reply | Report Abuse | Link to thiswith the astounding advent of life , i think it has a fundamental universal form of origin.
of course if we could find alien life on Mars and Europa it might tell us if there is one possible progenitor of life.
@vendicar9: it's cold, but there are also hot spots where extromophiles might be found. The gravitational forces on Europa cause significant crust migration and friction which leads to areas highly conducive to life. We're most likely not going to find life all over Europa*, but it appears all of the basic building blocks are there, there's spots where it's warm enough...
Reply | Report Abuse | Link to this*Although, we could. Life on Earth has diversified enough that there are extremophiles that live in extremely cold areas on the planet. Why couldn't life emerge in the hot spots and then migrate to the colder areas?
The book "Rare Earth" makes an educated, albeit speculative, argument for life in the universe being fairly common in solar systems with stable enough stars (G or K class--hotter stars get hotter too quickly; cooler ones "burp" periodically, nuking nearby planets) in single star systems with a rocky planet that has a magnetosphere, located in the solar system's life zone (i.e. where water is liquid).
Reply | Report Abuse | Link to thisThat's for bacterial life, however. For more advanced life you need to throw in a big Moon like ours, the "Rare Earth" authors claim. It's needed to give the planet axial stability, without which the long-term weather systems would be too extreme for advanced life. And throw in a Jupiter-class gas giant farther out to "vacuum" up asteroids and comets, keeping the asteroid strike count down to a dull roar, of course.
Still, with hundreds of billions of stars in hundreds of billions of galaxies in our universe alone (and there may be who knows how many other anthropogenic universes), the simple odds of Earth's unusual set of circumstances repeating millions and millions of times is, well, astronomical.
The problem is how far. And there, unfortunately, is the kicker. Even if every single other solar system in the universe had intelligent life, sublight travel (the only kind there is in all likelihood) would make it a minimum 60,000 year or so round trip even to/from Alpha Centauri.
So the very best we can hope for is radio contact--and even there, say the nearest technical civilization is, oh, 40 light years from here. So I say "Hi; they reply promptly, but I've died before hearing back. Some conversation.
So whether we're unique or not we're probably stuck with ourselves. To think that means we're the only ones out there though...well, that's the actual leap. Not the other way around.
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@ antonl - I'm not sure if this entirely rules out panspermia... maybe not that 'life' was seeded from other planets, but what about the building blocks of life.
Reply | Report Abuse | Link to thisI think it was pretty recent that researchers found amino acids on a comet... Check out this link for a cool video on this stuff. http://www.scienceinseconds.com/video.php?vId=117&tId=
May I clarify the discussion somewhat?
Reply | Report Abuse | Link to thisThe article as well as Darwin's books discuss the origin of species, not the origin of life.
We know a lot about evolution and the development of new species from older forms.
But we know nothing about the origin of life at all. Except, maybe, that it seems very unlikely that life could start at all.
But it has somehow happened.
But friends, this is a completely different story.
Your assertion that "we" (and unfairly including the entire discipline of science along with our own ignorance) know nothing about life's origin is utterly incorrect. For example, we know quite precisly the abiotic paleoenvironmental conditions. We know the time. What we don't know undoubtedly is greater than what we do know, but the exciting thing is not the retelling of a all-too-familiar old myth. What is exciting to the scientist is the question of what happened in between those early abiotic, anaerobic, radiated, NEO bombarded conditions, and the RNA-dominated code of life that emerged shortly after. The exciting question is precisely how was the code for life replicated before RNA? There are many possible answers, all of which are fascinating studies, and all of which may have played a part.
Reply | Report Abuse | Link to thisOne problem is that we cannot run the experiment over again. However, in case you refuse to ponder one profound logical extension - that doesn't mean it never happened.
nice article...i hope someone(s) follow up and either proves it or disproves it or qualifies it....sounds like its a first step....
Reply | Report Abuse | Link to thisEvolution is for those who refuse to eccept the obvious.
Reply | Report Abuse | Link to thisGod created us we are his children.
Do not be fraid of the truth. Talk to Jesus!
bigbaddude...probably should change your name to smallscaredchild.....
Reply | Report Abuse | Link to this"Junk science" is a term employed by industry-sponsored climate change deniers and the dupes who echo them, not by reputable scientists.
Reply | Report Abuse | Link to thisCurrent theories of global climate change are supported by a wide range of empirical data and not solely by the results of computer modeling.
Nevertheless, models have been very successful in reproducing observed changes in global climate.
My guess: you aren't in the least qualified to critique the models or the science of global warming. You're just blowing smoke based on right wing political ideology.
Give it a rest. You're only singing to the ultraconservative choir.