I have a fondness for graphs, especially ones that let you survey the sweep of life’s history in one glance. Here’s a new one, out today in the journal Science. It’s the latest look at the levels of biodiversity over the past half billion years. Scientists crunched the numbers on about 3.5 million fossils of ocean-dwelling invertebrates, using more detailed data than in previous surveys. (Some marine invertebrates leave fabulous heaps of fossils. Other species, like our own, are more delicate.) The horizontal axis marks time, and the vertical one marks the number of genera alive at any particular interval of about 10 million years. (Genera are groups of species. The genus Homo includes us, Neanderthals, and a few other extinct hominids, for example.)
The world, according to this graph, is a more diverse place than it was 500 million years ago. Diversity rose, then dropped during mass extinctions, then rose again. Some of the sexiest mass extinctions, like the one that claimed the dinosaurs 65 million years ago, turn out not to have been that big of a deal for marine invertebrates. Previous versions of the graph had suggested that since that die-off there had been a tremendous exponential burst of diversity, but now things look different. The genera count was rising a lot faster before the dinosaurs disappeared than after.
In fact, the overall rise of diversity has been quite modest. In the past, scientists have explained the rise of diversity as an exponential process, but if that’s true, then something has been keeping it in check. Perhaps there isn’t enough energy to let evolution run wild. The energy that comes from the sun gets dissipated as it flows through complicated food webs, and so it can’t support lots of new genera.
That’s why big-picture research on evolutionary biology is so important–unless you can step back, you may not be able to see the elephant in the room.
[Update: Edited with help from the commenters]
July 3rd, 2008 at 6:24 pm
>Perhaps there isn’t enough energy to let evolution run wild. The energy that comes from the sun gets dissipated as it flows through complicated food webs, and so it can support lots of new genera.
Should that be “CAN’T support lots of new genera.”?
That would seem to be more in keeping with limited energy keeping the exponential growth in check.
July 3rd, 2008 at 6:51 pm
Indeed. Fixed it. Thanks!
July 4th, 2008 at 3:02 am
Fascinating! Looks like there’s a couple of sharp drops after gradual rises.. But what’s up with the gradual decline in the Carbonian?
July 4th, 2008 at 3:38 am
Intriguing, yes…but greater diversity now than millions of years ago does not mean (as you know) that we have any excuse to ignore the destruction of ecosystems across the globe, with concomitant extinctions, that we carry out in the name of progress.
July 4th, 2008 at 5:29 pm
Everyone interested in biological diversity should read this great article on still more evidence that a severe drop in diversity has already began.
July 5th, 2008 at 3:32 pm
Interesting! The bin size is a bit too big to see any periodicity on a, say, 60 million year time scale. There’s a claim that there is a periodic drop in number of genera on that scale, possibly linked with the Sun’s orbit around the Milky Way. I explore this a bit in my upcoming book, and while it’s interesting, it’s still tentative.
July 7th, 2008 at 1:33 pm
Hmm, this graph seems to contradict a claim I read in a book by Peter Ward (The Life and Death of Planet Earth, I think) that earth’s total bioproductivity had probably peaked during the Permain and has been dropping steadily since.
July 7th, 2008 at 8:57 pm
WARNING: nit-picking ahead…
Shame on you, Mr. Zimmer! As one of the best and a leading science journalists out there (and, might I add, one I respect a great deal), I find it surprising that you would perpetuate this long-standing myth about the Cretaceous-Paleogene extinction event! Surely you know that birds are extant dinosaurs (birds evolved from theropod dinosaurs and therefore are theropod dinosaurs, just as humans evolved from primates and therefore are primates), so the quoted sentence is patently false as stated. Is it true that the end-Cretaceous extinction claimed most of the dinosaurs? Certainly. Is it still a “sexy” extinction event (in the sense of being evocative for the non-specialist and attracting lots of scientific attention)? Of course. But we’ve known for over 20 years now that dinosaurs did not go extinct at the end of the Cretaceous because some of them survived and even thrived in the Cenozoic and supercede mammals in terms of diversity today! Please…let’s kill this myth that dinosaurs went extinct at the end of the Cretaceous!
July 7th, 2008 at 9:43 pm
I let that one slip by me. When I say dinosaurs, I mean, of course, non-avian dinosaurs.
July 8th, 2008 at 10:48 pm
WARNING: using loupes to pick at even smaller nits. . . .
one that claimed the dinosaurs
and
this myth that dinosaurs went extinct at the end of the Cretaceous
That event certainly claimed dinosaurs. Lots of them went extinct. A vast majority, I would presume. Mr. Zimmer did not write “all” dinosaurs.
July 9th, 2008 at 12:50 pm
Would we not expect an explosion of diversity shortly after a new phylum emerged as new niches are exploited and filled, and a slow-down in diversity once available niches are evolutionary innovation’s outer limit?
July 9th, 2008 at 12:53 pm
Are BEYOND evolutionary innovation’s outer limit, that should say.
OK, and I didn’t do a great job with the whole comment. What I’m trying to get at is, I picture the first air-breathing tetrapod and there’s all this LAND with all these various terrains and climates and such, and evolution can go off wildly in many directions. But once those niches are filled, I would think that the capacity for radical innovation would taper severely. Could that not be the limiting factor?