ASCB’s joint meeting in Philadelphia with the International Federation for Cell Biology (IFCB) last month was all about crossing borders. Twenty-eight percent of the attendees were from institutions outside the U.S. while another harder to count but strongly represented group came from biophysics, bioengineering, mathematical modeling, and other physical sciences. The newcomers were there because their research paths were leading them deeper into the cell world. At the Saturday, December 6, keynote session, ASCB President Jennifer Lippincott-Schwartz welcomed them all, the international and the interdisciplinary crowd along with cell biologists at all levels from grad students to senior faculty, saying, “This looks like a big meeting but ASCB is really a galaxy of small meetings. Here you can link up with peers but you can also see, hear, and question the leaders in your field. Many of the names that you know well from citations are here at this meeting, walking around, drinking coffee, looking for someone to talk with. Don’t be shy.”
The two featured keynote speakers, astronomer Steve Squyres of Cornell and earth scientist Robert Hazen of the Carnegie Institution of Washington’s Geophysical Lab, represented an unusual corner of the ASCB/IFCB galaxy. The speakers looked out at an audience of biologists and promptly launched into geology, Earthly and Martian. The common theme from their different perspectives was the complicated dance between living and non-living materials.
The first keynote started on Mars. Squyres, a planetary astronomer at Cornell, is the PI of NASA’s Mars Exploration Rover (MER) mission. After a career studying Martian geology largely as a theoretical subject, Squyres now contends with the nuts and bolts of exploratory robots navigating a treacherous and hostile terrain. The MER mission is to reveal the hidden geology of a truly alien landscape but a basic question underlying this 12-year-old mission, according to Squyres, is whether there has ever been organic life on Mars and, if so, does it persist today somewhere on or beneath the surface? That is of immense interest to terrestrial biologists because it confronts the question of the universality or the unique occurrence of life on Earth. Said Squyres, “It (Mars) is a terrible place. It’s a cold and dry desolate world today. In the past, it was different. It was warm and wet at the same time in Earth history that life was developing on earth,” That warmer and wetter ancient Mars offers biologists the chance to wind time backwards and watch an alternative biological history of Earth, according to Squyres.
Replaying the tape of time was central to Hazen’s talk on what he calls the “co-evolution” of living and non-living matter on Earth. This is not the same as Darwinian evolution with descent from a common ancestor but Hazen, a mineralogist and astrobiologist, contends that the diversity of mineral “species” on Earth—4,957 of them have been described—would not be possible on a planet without an active and reactive biosphere. Hazen said, “The diversity and distribution of minerals on Earth today is a consequence of both deterministic and stochastic physical, chemical, and, most importantly, biological processes. If you play the tape of Earth’s history over again, you’d have different minerals.”
Hazen began his case in deep, deep time, 4.6 billion years ago, in the pre-stellar dust that contained all the chemical elements but no more than a dozen minerals. Through the slow accretion of planetary disks where compression and heating increased, mineral diversity on the proto-Earth grew to about 250 mineral species. On and inside a solidified Earth the new forces of volcanism, degassing, fractional crystallization, igneous diffusion, and plate tectonics brought the total to around 1,500 mineral species. And here began the biological mediation of Earth. “The core of our hypothesis is that we are basically saying two-thirds of all mineral species arise from this Great Oxidation Event and subsequent biological innovations,” said Hazen.
He divides biological and mineral co-evolution into five stages: the appearance of anoxic biota 3.9-2.5 billion years old that stripped vast quantities of iron and carbonates from the oceans; the rise of cyanobacteria 2.5-1.9 billion years ago, which set off the Great Oxidation Event and began incorporating minerals into biological structures; the “boring billion years” of the Intermediate Ocean, 1.9-1.0 billion years ago, where microbes quietly carried out sulfide reduction in the seas; “Snowball Earth” 1.0-5.7 billion years ago when biologically driven oxygen pulses set off violent alternating global glaciation events; and our time, the Phanerozoic Era where biological processes through oxidation, weathering, and biomaterial incorporation dominate the evolution of new minerals. The result is 4,937 known mineral species today.
Using analytical methods borrowed from ecological modeling, Hazen said that we can quantify the relative rarity of a planet like ours. “In mineralogy, we have data on roughly 5,000 mineral species and 125,000 localities. We have 635,000 distinct mineral locality data points so we can do species frequency distributions. These are quantitative relationships. What I am going to argue is that the distribution of those 5,000 minerals shows a rare event distribution similar to that of words in a book or biomass in a ecosystem.” Books have lots of words but when you cull out the common ones, you are left with a handful of rare ones that are distinctive to that one book. If you analyze the location and relative rarity of the Earth’s mineral species, you would find a similar pattern of rare-event distributions. Fully 22% of all mineral species can be found in only one locale, said Hazen. This rare-event distribution identifies the Earth as surely as any individual’s fingerprints or genome. With it, we can make specific predictions of how often Earth’s mineral diversity is likely to occur on another planet. “We can replay the tape,” Hazen said and has, at least, mathematically. “The chance of reproducing Earth’s exact minerals, if we replayed the tape, is less than one in 100200.”
This demonstrates, Hazen told his keynote audience of biologists and nontraditional biologists, Americans and international colleagues, and beginning students and world-renowned experts that, “Earth is unique in its biosphere largely because of this co-evolutionary tug of war between biological forces and the minerals of Earth.”
In short, there can be no place quite like our blue, green, and co-evolving planet.