Monday, 1 November 2010

The Complexities of Life

From New Scientist: Why complex life probably evolved only once.
The universe may be teeming with simple cells like bacteria, but more complex life – including intelligent life – is probably very rare. That is the conclusion of a radical rethink of what it took for complex life to evolve here on Earth.

It suggests that complex alien life-forms could only evolve if an event that happened just once in Earth's history was repeated somewhere else.

All animals, plants and fungi evolved from one ancestor, the first ever complex, or "eukaryotic", cell. This common ancestor had itself evolved from simple bacteria, but it has long been a mystery why this seems to have happened only once: bacteria, after all, have been around for billions of years.

The answer, say Nick Lane of University College London and Bill Martin of the University of Dusseldorf in Germany, is that whenever simple cells start to become more complex, they run into problems generating enough energy.
So if Lane and Martin are right, the textbook idea that complex cells evolved first and only later gained mitochondria is completely wrong: cells could not become complex until they acquired mitochondria.

Simple cells hardly ever engulf other cells, however – and therein lies the catch. Acquiring mitochondria, it seems, was a one-off event.

And from Transterrestrial Musings, a stream-of-consciousness report about Craig Vetnor's presentation at
Has synthesized a megabyte chromosome. Everything in the cell was derived from the chromosome and the natural traces were all deleted. They are digitizing biology. Converting analog genetic code to digital. Now they can go the other way, from ones and zeros to living organisms. Huge progress over past two decades. Big breakthrough new algorithm in 1995. New approach to sequencing pieces by breaking them down and putting into the computer....
Yes, and compilers for manufacturing prokaryotes (organisms with no cell nucleus), and later assembling eukaryotes (organisms with a nucleus). Arranging for the organism to make its own membranes has been surprisingly difficult - though programming it has been surprisingly easy.

In terms of cell numbers - your body has ten times as many cells without your DNA in, than with. But the cells with your DNA in are larger and more complex than the swarms of bacteria to which you play host, so most of your mass is you.

A bucketful of seawater has more genetic diversity inside its bacteria than all the complex organisms on the planet put together - and they're constantly swapping DNA in a promiscuous fashion, so new "species" are being created all the time - though the concept of "species" tends to lose its meaning under those circumstances. From Scientific American:
In fact, the bacteria in the wild—the researchers tested microbes in coastal, estuary, reef and open ocean environments—were quite promiscuous with their DNA, busily transferring genes to not only their own species but also other closely related bacteria and even other genera. They were also doing it thousands to hundreds of millions of times more frequently than previously estimated for other methods of gene transfer, such as via phages or bacterial viruses (the method also employed by human gene transfer agents, also known as synthetic biologists).
Nevertheless, such horizontal gene transfer is a potent weapon in the bacterial evolutionary arsenal. "Truly novel genes can be taken on instantly. This is why we have antibiotic-resistant superbugs," Doolittle says. In fact, sequencing bacterial genomes has revealed that such gene transfer is responsible for many of the genes present in today's microbes. "Many already accept that HGT is very important to marine microbial adaptation."

And not just for bacteria. Given the apparently vast numbers of such genetic packets—and viruses, plasmids and other bits of genetic material—the ocean can be seen as a bit of a microbial DNA soup.
One off? Maybe not.


Lloyd Flack said...

There is no genetic evidence of eukaryotes evolving more than once. They could not evolve until there was enough oxygen available for aerobic metabolism. When they did evolve I would expect them to have diversified until they took up all the available niches for such organisms, pre-empting any second evolution of complex cells.

Zimbel said...

While I agree that eukaryote almost certainty evolved only once, I'd make a stronger statement - it seems that prokaryotes may also have evolved only once; more to the point, we have little evidence that they didn't.

Where is my claimed evidence or lack thereof? In the genetic codes themselves. Even in genetic codes that are widely disparate (say the Yeast Mitochondrial code versus the human nuclear code), there are striking simularities.
In that particular case, 8 of the 64 codons code for something different than each other; random chance would suggest far higher numbers.

So one of three things is making these genetic codes similar:
1) Our sample is biased towards similar genetic codes - i.e. we haven't found vastly different ones yet.
2) These codes are heavily related to each other, likely coming from a common ancestor.
3) The chemistry itself is highly biased towards specific genetic codes. Note that there is evidence of this.

I tend to favor #2 and #3, although obviously a future discovery of a very different genetic would disproove #2.

A site on genetic codes, updated a few months ago: The Genetic Codes Compiled by Andrzej (Anjay) Elzanowski and Jim Ostell National Center for Biotechnology Information (NCBI), Bethesda, Maryland, U.S.A.

mythusmage said...

You have read up on the myxobacteria

Lloyd Flack said...

One think that could give us an indication of the likelihood of the development of complex cells is how soon did they develop after sufficient oxgen became available. The more likely their development was, the sooner they would probably have developed.

The oldest eukaryotes that we are certain of are from 1.2 billion years ago. There are possible eukarote fossils from as much as 2.1 billion years ago. There are possible biomarkers from 2.7 billion years ago.

If the last figure is correct then they probably evolved nearly as soon as they could have. If they evolved considerably later then they could well have originated in a fluke event.

Zimbel said...

Some of the most disparate Genetic codes are from Mitochondria. I think perhaps a good way to try to date Eukaryotes would be to try to map out mitochondrial DNA variance, much as doing so in Humans alone gives us a guess on Mitochondrial Eve.

I have no idea if such an approach is feasible, though; my Biology is archaic, particularly in the area of genetics. There are a lot more mitochondrial generations than human ones, particularly along certain lines.

mythusmage said...

I get the feeling certain parties don't understand what myxobacteria are. In a word, myxobacteria are multicellular life forms, slime molds in short, based on bacteria instead of eukaryotic cells. Bacterial slime molds, with all the features of standard slime molds. Including cell differentiation, fruiting bodies, and specialized cells for procreation. When you consider how little it took to get one eukaryotic slime mold recognized as a primitive animal, the leap to a bacterial animal isn't that hard for myxobacteria to make.

Anonymous said...

Carl Sagan would disagree. When numbers are so large and time an unsure factor anything can happen, it is called the unknown. We do not have all the factors to make good datum. So play modleing all you want something is still missing.


Zimbel said...


I'll admit my ignorance as to myxobacteria. However, from the reference you cited, it looks like they are bacteria that swarm, work together, and specialize. In other words, they have some of the features of multi-cellular life forms, but not all - for example, there aren't specialized tissues. The analogy you make with slime molds is a good one.

In any case, your taxonomy is incorrect; neither is in the kingdom Animalia. Slime Molds are usually classified as Excavata (or Acrasidae - depends on who you ask), Chromalveolata, or Rhizaria (depending on the kind), whereas myxobacteria are Bacteria.

@anonymous - Carl Sagan? The same astronomer who loves the Drake equation (which itself is a model with multiple poor inputs)? I think he likes models, he might just disagree on the model or its inputs.