Old views about evolution of life challenged
Washington: Researchers have debunked a popular hypothesis behind the evolution of early life on Earth.
Scientists led by biogeochemists at the University of California, Riverside tested a popular hypothesis in paleo-ocean chemistry, and proved it false.
The fossil record indicates that eukaryotes - single-celled and multicellular organisms with more complex cellular structures compared to prokaryotes, such as bacteria - show limited morphological and functional diversity before 800-600 million years ago.
Many researchers attribute the delayed diversification and proliferation of eukaryotes, which culminated in the appearance of complex animals about 600 million years ago, to very low levels of the trace metal zinc in seawater.
As it is for humans, zinc is essential for a wide range of basic cellular processes. Zinc-binding proteins, primarily located in the cell nucleus, are involved in the regulation of gene transcription.
Eukaryotes have increasingly incorporated zinc-binding structures during the last third of their evolutionary history and still employ both early- and late-evolving zinc-binding protein structures.
Zinc is, therefore, of particular importance to eukaryotic organisms. And so it is not a stretch to blame the 1-2-billion-year delay in the diversification of eukaryotes on low bioavailability of this trace metal.
After analysing marine black shale samples from North America, Africa, Australia, Asia and Europe, ranging in age from 2.7 billion years to 580 million years old, researchers found that the shales reflect high seawater zinc availability and that zinc concentrations during the Proterozoic era were similar to modern concentrations.
Zinc, the researchers said, was never bio-limiting.
"We argue that the concentration of zinc in ancient marine black shales is directly related to the concentrations of zinc in seawater and show that zinc is abundant in these rocks throughout Earth's history," said Clint Scott, the first author of the study, adding "we found no evidence for zinc biolimitation in seawater".
Scott said that the connection between zinc limitation and the evolution of eukaryotes was based largely on the hypothesis that Proterozoic oceans were broadly sulfidic.
Under broadly sulfidic conditions, zinc should have been scarce because it would have rapidly precipitated in the oceans, he said.