Here’s a problematic graph:
See how that line running from left to right extends behind the vertical line that marks the origin of Earth? Basically, what this graph posits, that if life becomes more complex at a steadily exponential rate that can be extrapolated from recent evolutionary history, then life predates Earth.
It comes from a thought-provoking paper published at arXiv (a non-peer-reviewed journal that’s a favorite of mathematicians and physicists for pre-press papers), from which Alexei Sharov, a staff scientist at the U.S. National Institute on Aging, and Richard Gordon, a theoretical biologist at the Gulf Specimen Marine Laboratory in Florida. The pair plotted “genome complexity,” measured by the genome size of major phylogenetic trees, on a logarithmic scale over time. The resulting graph, shown above, determines that life gets doubly more complex about every 376 million years.
Sharov and Gordon said the relationship reminded them of Moore’s law, which describes how computers increase in complexity exponentially. Moore’s law holds that the number of transistors on a computer processing unit will double every two years (sometimes given as 18 months), accounting for an exponential growth in processing power.
If you use Moore’s law to extrapolate backward from modern computers, you’ll get back to zero in the 1960s, when microchips originated.
But if you apply the same logic to the genomic data (going back to a genome size of just a single DNA base pair) the equation doesn’t seem to square -- life would have to originate around 9.7 billion years ago. The Earth itself is only 4.5 billion years old. On its face, the graph seems to argue for the development of life on other planets -- did our pre-cellular ancestors ride here on the back of an asteroid?
There are several problems with this theory, though. The major one is that, obviously, we don’t have any 9.7-billion-year-old fossils to confirm what’s projected on the graph. As XKCD -- the popular nerdy webcomic -- illustrates, extrapolating beyond your data can be a dangerous thing.
The “origin point” on the graph, with a lifeform consisting of just a single DNA base pair, is also not really likely to have ever existed. Many scientists think that the early forms of life on Earth had genomes made of RNA, or possible some other kind of nucleotide.
What might really raise the eyebrows of biologists is the fact that Sharov and Gordon used genome size as a measure of “genetic complexity.” The largest known genome on Earth belongs to either the amoeba Polychaos dubium (possibly 670 billion base pairs of DNA long, though this claim is disputed) or a rare Japanese flower called Paris japonica (149 billion base pairs of DNA). Humans (3.2 billion DNA base pairs) don’t even rank highest among vertebrates; that honor goes to the marbled lungfish, which has a genome made of 130 billion base pairs of DNA.
Also, Moore’s law of doubling complexity applies to the development of microchips; it wasn’t designed to describe the behavior of a messy biological process like evolution.
Evolution as we understand it now is not a linear path of developing complexity; the theory of punctuated equilibrium assumes a more chaotic process full of spikes and dips. Short periods can see a furious radiation of evolution and speciation, thanks to a number of factors. A certain trait (like the ability to breathe air) could arise and allow organisms to colonize new territory, or some massive extinction event (like the asteroid blamed for the end of the dinosaurs) clears the board for a new set of species to take over. Some organisms change rapidly over the generations; others, like the coelacanth, are content to stay pretty much the way they are for millions of years.
“Is it reasonable to think that the complexity of life has increased at the same rate throughout Earth’s history?” the arXiv blog on MIT’s Technology Review wondered. “Perhaps the early steps in the origin of life created complexity much more quickly than evolution does now, which will allow the timescale to be squeezed into the lifespan of the Earth.”
SOURCE: Sharov et al. “Life Before Earth.” arXiv:1304.3381.
Roxanne has liked science ever since she started watching "Bill Nye the Science Guy" on Saturday mornings over a bowl of sucrotic O's. She especially likes writing about...