Two billion years ago the green scum of cyanobacteria and their ilk began to rule Earth. During their billion-year reign, our cellular ancestors, the eukaryotes, went nowhere evolutionarily. Now a geochemist and a paleontologist propose a provocative explanation: They may have been too hungry to diversify.
The hypothesis, published in the 16 August issue of Science by geochemist Ariel Anbar of the University of Rochester in New York state and paleontologist Andrew Knoll of Harvard University, is founded on a 1998 proposal by geochemist Donald Canfield of Odense University in Denmark. Canfield suggested that almost all water in the oceans remained oxygen-free, or anoxic, after oxygen first appeared in the atmosphere about 2.2 billion years ago. Drawing on measurements of the sulfur-isotope composition of ancient ocean sediments, he argued that the newly oxygenated atmosphere weathered large amounts of sulfur off the land and into the ocean, where it took the form of sulfides. By forming insoluble compounds, these sulfides removed most of the iron from the sea.
Eukaryotic algae would have sorely missed the iron, and other metals--including molybdenum, copper, zinc, vanadium, and cadmium--lacking in the Mesoproterozoic ocean, 1800 million to 800 million years ago, Anbar and Knoll argue. Their enzymes for taking up essential nitrogen in the form of nitrate are built around an iron atom and a molybdenum atom. All in all, the Mesoproterozoic oceans offered only poor nutrition and hard times, the pair concludes.
Eukaryotic algae would have been at an evolutionary disadvantage. Multicellular algae, in particular, compete best when levels of nitrate are high, not just at the bare minimum. That could explain the persistently low diversity of eukaryotic algae through the Mesoproterozoic, Anbar and Knoll say. Only after mountain-building wrenched North America, sending more weathered metals into the sea, and atmospheric oxygen levels increased further, converting sulfides to sulfates and freeing up the trace metals, could the eukaryotes diversify toward larger, multicellular plants.
The idea "is elegant," says geochemist Timothy Lyons of the University of Missouri, Columbia. "It explains a lot, but we really need to have more data. This paper will define research for years to come."