Life or Complex Life?

[CDarwin]

Is it more likely that life per se should evolve or that "complex" life (plants and animals or their equivalent) would evolve (once life has) on any given planet? Don't try to add up the probabilities. We're comparing the chances of life evolving with the chances of complex life evolving on any planet where life is.

 

This is a question posed by Carl Sagan in his Cosmos (sort of). He seems to take the position that complex life is more the more unlikely of two. I do as well. The mechanisms for the origin of the simplest 'life' seem fairly straightforward. It's just chemistry really. From there natural selection should take any primitive cell on a fairly automatic path of tightening up replication mechanisms and bolstering its abilities to survive and reproduce.

 

Evolution of "higher" lifeforms seems like it would require a more specific set of circumstances that might be hypothesized to be rather rare in the universe.

 

Of course this is speculation piled on speculation, but I think we can at least approach the question.

 

EDIT: Ah, I meant to make the poll public. Oh well. I don't suppose it matters that much.

[Realitycheck]

If humans are taken out of the equation on earth, how much longer would it take primates to evolve to where we are today? If we take primates out of the picture, is it feasible that any other family or genus could come anywhere near? I doubt it, but the possibilities are really quite widespread when you factor in another billion years of development.

 

It's really a tough call, many variables, though there are some specific requirements that need to be met in order to achieve that hurdle, definitely not something that happens everywhere.

[CDarwin]

If humans are taken out of the equation on earth, how much longer would it take primates to evolve to where we are today? If we take primates out of the picture, is it feasible that any other family or genus could come anywhere near? I doubt it, but the possibilities are really quite widespread when you factor in another billion years of development.

 

If primates were out of the picture than squirrels and birds would probably radiate, would be my guess. That certainly doesn't mean we'd have super smart squirrels or birds, though. Humans have brains as large as they do because of a long chain of specific circumstances in a unique history that is unlikely ever to be exactly replicated. That doesn't mean there aren't plenty of other evolutionary paths to human-like intelligence, of course.

[YT2095]

I would say simple life, single cell stuff 1`st.

then maybe given enough time these cells may find it beneficial to work in a cooperative manner and perhaps form complex life.

[thedarkshade]

I'd say complex life is still life, but lie YT I too think that there must be a starting point, a cell from which more advanced life forms could possibly evolve. But yet, there is not reason why life in any other planet has to be similar to our form of life.

[Mr Skeptic]

I vote that life --> complex life is far more likely than non-life --> life. That's because life itself is already extremely complex, and once you have life all bets are off as to what it can do. Also, the universe currently shows 1 planet with life and complex life, no other planets with life. I'm surprised everyone else thinks life is easier than turning life into complex life. Maybe they misunderstood the question?

[Daecon]

How do you define "complex"? Surely life by it's very nature is already complex?

[thedarkshade]

Surely life by it's very nature is already complex?

Yes, but we have more and more complexity in life as we move to 'higher levels' of evolution. Therefore the most primitive organism we know could me considered as the simplest form of life.

[Sayonara]

I would vote for the second option, on the basis that life beginning in the first place is currently considered quite uncommon, whereas becoming more complex once established is basically what life does.

[Realitycheck]

I'd like to see a grizzly bear assemble a circuit board, even after he somehow becomes smart enough to figure out how to do it. So even if they somehow developed a large brain, they would not be able to use it because they would be physically unable to accommodate the use.

[CDarwin]

How do you define "complex"? Surely life by it's very nature is already complex?

 

I already did. Plants and animals or their like.

[Daecon]

I agree with Sayonara³. Once life becomes established, I would expect evolution to take over almost instantly and drive the competition...

[CDarwin]

I agree with Sayonara³. Once life becomes established, I would expect evolution to take over almost instantly and drive the competition...

 

But would that competition lead inexorably to "higher" life? For the first billion years or so of life on Earth, evolution never saw fit to originate anything but microbes.

[Mr Skeptic]

But would that competition lead inexorably to "higher" life? For the first billion years or so of life on Earth, evolution never saw fit to originate anything but microbes.

 

That seems probable. Even cooperating together/against others is useful, so is forming colonies. Once you have colonies, being mulitcellular is an obvious next step.

[Sayonara]

But would that competition lead inexorably to "higher" life?

Let's stick with complex, which was specified in the OP. "Higher" is a bit more subjective.

 

For the first billion years or so of life on Earth, evolution never saw fit to originate anything but microbes.

True (as far as we can evidence), but no time scale restriction was given in the OP.

[CDarwin]

Let's stick with complex, which was specified in the OP. "Higher" is a bit more subjective.

 

Mm, I used both "higher" and "complex" in the OP. They're both reasonably subjective terms. I'm just talking about plants/animals/similar.

 

True (as far as we can evidence), but no time scale restriction was given in the OP.

 

But why did complex life only evolved on earth when it did after the specified time period because of specific environmental changes that were going on at the time. Would the conditions wrought by these changes exist in the frequency that environments necessary for the origin of life itself might?

 

Look, for another example, at the enormous range of habitats that microbial life has conquered. You can go from salt lakes to hydrothermal vents. Complex life inhabits only the most verdant and hospitable corners of the most verdant and hospitable world yet observed.

[Realitycheck]

I am thinking that it took a long time for the plankton to produce the oxygen necessary to enable land-bound animals. That doesn't explain restrictions for marine animals though. Water is water. What else would take so long to accomplish that would enable the development of marine life? Why such limited mutations over 3 billion years, then we go from sponges to dinosaurs in only a few hundred million years?

 

It almost appears that the genome needed that long to grow to accommodate things like dinosaurs. What I am reading now suggests that the first protocells used simpler peptides rather than RNA or DNA as their nucleic acids, leaving lots of time to develop. Mycoplasma genetalium only has 300-350 genes, suggesting that maybe it developed or was parented way back during this Pre-Cambrian era.

[Sayonara]

Mm, I used both "higher" and "complex" in the OP. They're both reasonably subjective terms. I'm just talking about plants/animals/similar.

So you did... my error. I am thinking about this in the same "plants/animals/similar" terms as yourself.

 

But why did complex life only evolved on earth when it did after the specified time period because of specific environmental changes that were going on at the time. Would the conditions wrought by these changes exist in the frequency that environments necessary for the origin of life itself might?

I suspect that once life becomes established in a resource-rich environment, progressive evolution is simply a numbers game. Selection drives adaptability, but there will inevitably be a long period of time before the first tipping point where one advantageous adaptation causes a leap forward in competitive strategies. This is because of the relative simplicity of the information structure and methods of transmission in primitive organisms.

 

Look, for another example, at the enormous range of habitats that microbial life has conquered. You can go from salt lakes to hydrothermal vents. Complex life inhabits only the most verdant and hospitable corners of the most verdant and hospitable world yet observed.

I wouldn't call hydrothermal vents or salt lakes hospitable, but then I am not adapted to them . What we learn from ecosystems adapted to these environments is that given enough time, living systems can adapt to exploit resources even if the conditions where those resource are found are atypical in some extreme or another. That in itself is good evidence (although rather circumstantial, I have to admit) that once life arrives, it can adjust its complexity as it encounters new survival challenges. Such challenges might even be the specific environmental changes you mentioned earlier, but they do not have to occur on a planetary scale or across thousands of years - since a single population can evolve, and single populations are very vulnerable to changes in resource levels or abiotic conditions, they just have to happen, and further complexity results. Or, you know, extinction. Depends on the circumstances!

 

Why such limited mutations over 3 billion years' date=' then we go from sponges to dinosaurs in only a few hundred million years?

It almost appears that the genome needed that long to grow to accommodate things like dinosaurs.[/quote']

I think we almost certainly oversimplify, but we agree.

[Realitycheck]

Interestingly, "primitive" cyanobacteria already have one chromosome but with 6,300 genes. That's a big molecule.

[CharonY]

I am thinking that it took a long time for the plankton to produce the oxygen necessary to enable land-bound animals. That doesn't explain restrictions for marine animals though. Water is water. What else would take so long to accomplish that would enable the development of marine life? Why such limited mutations over 3 billion years, then we go from sponges to dinosaurs in only a few hundred million years?

 

First, bacteria where the first oxygen producers. Regardless of aquatic or terrestrial life, it appears that most more complex eukaryotes require oxygen to produce sufficent energy to sustain themselves. This is one example of a limiting factor. If, for some reasons, photosynthesis has not evolved, life would consist almost exclusively by bacteria or archaea, capable of anaerobic respiration.

 

It almost appears that the genome needed that long to grow to accommodate things like dinosaurs. What I am reading now suggests that the first protocells used simpler peptides rather than RNA or DNA as their nucleic acids, leaving lots of time to develop. Mycoplasma genetalium only has 300-350 genes, suggesting that maybe it developed or was parented way back during this Pre-Cambrian era.

 

Complexity of an organism does not translate to size of a genome. Bacteria for example have a higher gene content to mass ratio than any other organism. Also M. genitalium is an extraordinary bad example to make your point as this bacterium has a reduced genome because it adopted a parasitic life-style. Consequently copious genes have been deleted out of its genome.

[Realitycheck]

Thanks. The conclusion that I am looking for is whether or not the DNA molecule evolved to a larger size over a large period of time (billions of years) through mutations or if it was resolved in a rather short period of time to achieve what was the best workable solution for spreading life. I guess the first 3.5 billion years still leaves a lot of room to work with, trying out different combinations and so forth.

[lucaspa]

Is it more likely that life per se should evolve or that "complex" life (plants and animals or their equivalent) would evolve (once life has) on any given planet? Don't try to add up the probabilities. We're comparing the chances of life evolving with the chances of complex life evolving on any planet where life is.

 

Define "complex life". I think it highly likely that life would evolve to multicellular organisms -- which satisifies the scientific concept of "complex life".

 

What Sagan is really talking about is sapient species. What are the odds that a sapient species will evolve?

 

I think the odds are 1, but that the time frame could be very long.

 

However, once natural selection begins exploring the Library of Mendel (all possible genomes) it will eventually get to the wing that contains genomes for sapient organisms. That "eventually" might be a VERY long time, however.

 

The conclusion that I am looking for is whether or not the DNA molecule evolved to a larger size over a large period of time (billions of years) through mutations or if it was resolved in a rather short period of time to achieve what was the best workable solution for spreading life. I guess the first 3.5 billion years still leaves a lot of room to work with, trying out different combinations and so forth.

 

It's not the DNA molecule, but metabolism. It appears that the first 2 billion years of life on earth was evolving an efficient and robust metabolic system. The DNA molecule gets longer by gene duplication or translocation.

[immortal]

Well I too vote for the second one because once life gets started it never looks back. Even though I think that the nature had the same kind of challenges to create nonlife -----> life and life -----> complex life it is more likely that complex life can come from simple life. For example, one challenge was the end replication problem which arosed when circular chromosomes evolved into linear chromosomes (i.e. dna polymerase can not synthesize DNA of the other strand due to lack of free -OH group at the end of chromosomes) this would have been lethal as it causes fusion of chromosomes but nature as always had find solutions to problems and the solution was the enzyme telomerase and there are other mechanisms which have evolved to solve this problem.

 

 

Whether or not humans and plants are complex organisms I think eukaryotes are surely complex than prokaryotes. What makes an organism multicellular is that its ability to make cell to cell adhesions which are provided by tight junctions, belt and spot desmosomes which have proteins like desmoplakins and desmogleins in their matrix and gap junctions for effective communiction. I think similar primitive proteins might have played a role in making multicellular.

 

 

Having introns and exons which can produce different proteins using the same gene in different cells through exon reshuffling and splicing adds some complexity and genome size does'nt matter much. Complex regulation in mRNA processing using trancriptional factors, repressors and different ways of regulation of start and termination signals is quite complex.

 

On the whole through constant programmed genome modificatons due to stress by transposons making translocations and gene duplications it is quite likely that complex life can come from simple life.