Several readers called my attention to a new paper by J. William Schopf and colleagues in the Proceedings of the National Academy of Sciences (reference below; free download at link), a paper that has also gotten a great deal of attention in the press. Last week a journalist asked me to comment on it, but I was too busy then to read it. Now that I have, I’m not all that impressed. It’s a decent paper, and doesn’t fail the first test of a science paper—does it tell us something new?—but I don’t think it makes the case that’s gotten the press all excited.
What is that case? The authors claim that their finding—an example of extremely slow evolution (in fact, no perceptible evolution) in a sulfur-producing bacterium—constitutes a test of Darwin’s “null hypothesis of evolution.” That hypothesis, as stated by the authors, is this:
. . . if there is no change in the physical-biological environment of a well-adapted ecosystem, its biotic components should similarly remain unchanged.
In other words, if there’s no selection pressure on the organism, it will not evolve. The authors implicitly equate “change in the physical-biological environment of a well-adapted organism” with “selection pressure on that organism,” and hence with “evolution of that organism,” but that’s not correct. I’ll talk more about this below, but for the time being try to guess how organisms could still continue to evolve in a relatively unchanging environment. Let’s look first at the author’s data, which, they claim, shows no evolution taking place in more than two billion years.
What Schopf et al. did, which is good stuff, is to examine bacterial microfossils in two ancient Precambrian biota and then compare them with modern fossils living in a similar environment. They were concerned with sulfur-using bacteria living on the ocean floor. The oldest formation examined was the “Turee Creek Group Kazput Formation” in northern Australia, a formation that’s ancient—about 2.3 billion years old. That’s old, but it doesn’t take the prize for age, for the oldest known bacteria (cyanobacteria, formerly known as “blue-green algae”) come from about 3.5 billion years ago.
The second group of fossil sulfur-metabolizing bacteria are about 500 million years younger than those from Turee Creek: they’re from the 1.8-billion-year-old Duck Creek formation, also in Western Australia. So we have about 500 million years of potential evolutionary change intervening between the two fossil formations.
Finally, the authors compared fossil bacteria from these two formations with sulfur bacteria that are still with us: a community of sulfur-using bacteria discovered in 2007 in the seafloor off the west coast of South America. All of these bacteria are presumed or known to use marine sulfur compounds, first reducing them to hydrogen sulfide and then oxidizing them to produce elemental sulfur and sulfur dioxide.
So what are the similarities among these three types of bacteria that led the authors to suggest that they hadn’t evolved over 2.3 billion years? There are three. First, all three bacteria lived in communities not in shallow water, but in the sea floor in deeper waters. Paleontologists have ways of telling this, and you can read the paper if you want to know how.
Second, they all have similar morphology, forming long filaments of about the same size. Here is what they look like; the figures show both the ancient bacteria from both formations and the modern collection (“A”). They form long, cylindrical filaments of comparable size
Third, chemical analysis of the matrix around the fossil bacteria, and of the modern ones, showed that they all lived or live in anoxic (“oxygen free”) communities, produce the same sulfur isotopes, and yield a form of pyrite as a metabolic product. The authors conclude, probably correctly, that the metabolic pathways for sulfur use in all of these forms are similar
So have they remained evolutionarily static, as the authors argue? First, let’s review their claim about these bacteria:
Once subseafloor sulfur-cycling microbial communities had become established, however, there appears to have been little or no stimulus for them to adapt to changing conditions. In their morphology and community structure, such colorless sulfur bacteria—inhabitants of relatively cold physically quiescent anoxic sediments devoid of light-derived diel [daily] signals and a setting that has persisted since early in Earth history—have exhibited an exceedingly long-term lack of discernable change consistent with their asexual reproduction.
They also claim that not only did the morphology and biochemistry of these species remain unchanged, but they didn’t form new species, either, although the concept of bacterial “species,” as Allen Orr and I showed in our book Speciation, is a bit hazy.
But I think the author’s conclusion is premature, and for two reasons.
First, regarding the “null hypothesis of Darwinism,” I think that that notion is wrong—or at least incomplete. Even in an unchanging environment, organisms can still evolve in significant ways. If new mutations arise that adapt the species better to that unchanging environment, then we will have evolution. For example, the bacteria could evolve more efficient metabolism of sulfur. Alternatively, one could have mutations that simply allow the bacteria to divide faster, giving them a selective advantage over others. Neither of these would be a response to a changing environment, but could still cause evolution. Considerable retooling of the bacteria’s metabolism, DNA synthesis, and so on, could still occur as evolution experiments with various mutations.
There is in fact one natural experiment that showed evolutionary divergence in an unchanging environment. There are salmon in some West Coast rivers (pink salmon, as I recall), that form “year classes”: they have a two-year life cycle and come into the same rivers from the sea to breed. They are basically the same species of salmon, but long ago a few stragglers switched form breeding in “even” years to breeding in “odd” years. Since the breeding seasons of the two classes don’t overlap, they became instantly reproductively isolated from each other in a unique way: they couldn’t interbreed, but at the moment the year divergence began they were genetically identical; and they continued to live in essentially the same environment. Yet over thousands of years the even- and odd-year forms have diverged, to the extent that they are somewhat reproductively incompatible when bred together—the beginning of speciation. This shows that a eukaryotic species living in very similar environments can still diverge genetically, and begin the process of forming new species.
Second (and the authors note this in passing), there could be considerable internal biochemical evolution taking place that can’t be detected from simply looking at the fossil bacteria or seeing if they metabolized sulfur in similar ways. After all, bacteria are morphologically simple, and there’s simply not that much room for visible change, especially if you’re constrained to look at fossil bacteria in rocks. And it’s impossible to sequence the DNA of the fossil bacteria or grow them in the lab, so we can’t see how genetically and metabolically different they are.
This is not just a theoretical possibility, for biologists are well familiar with this phenomenon. We know of many cases of “sibling species”: distinct species of closely-related organisms that can’t be told apart through morphology alone, but have diverged considerably in their non-visible characters. For example, I worked on a group of 8 Drosophila species in which females couldn’t be told apart by just looking at them, even under the microscope. (Males differed very slightly in their genitalia). Yet despite their morphological conservatism, the species diverged profoundly in ecology and in their reproductive compatibility: they don’t like to mate with members of the other species, and in many cases the inter-species hybrids were either inviable or sterile. Genetic analysis showed considerable divergence in the DNA, including in important traits affecting reproduction. The problem is even more severe if you are constrained to look at fossils, in which only the hard parts become mineralized. Important differences in softer parts that could reflect genetic change (granted, not that much of a problem in bacteria) could be missed.
The lesson is that it’s dangerous to use fossils—even bacterial fossils—to conclude that evolution hasn’t occurred. And the “null hypothesis of Darwinism” is a bit dubious anyway. Yes, species probably change most rapidly when the environment is changing, but there’s no reason why environmental change is a sine qua non for evolution.
The paper by Schopf et al. does show an intriguing case of morphological and metabolic stasis over billions of years, and it probably does reflect a lack of environmental change. But what it doesn’t show is that evolution hasn’t occurred in these bacteria. To know that, we’d have to have them all alive to sequence their DNA and look at their physiology, reproduction, and so on—and that’s impossible for the fossil species.
Schopf, J. W., A. B. Kudryavtsev, M. R. Walter, M. J. Van Kranendonk, K. H. Williford, R. Kozdon, J. W. Valley, V. A. Gallardo, C. Espinoza, and D. T. Flannery. 2015. Sulfur-cycling fossil bacteria from the 1.8-Ga Duck Creek Formation provide promising evidence of evolution’s null hypothesis. Proc Nat. Acad. Sci. USA, online Early Edition. 10.1073/pnas.1419241112.