Trace metals like iron and copper became rare in the oceans between 2 and 1.2 billion years ago, after having been abundant for the previous billion years. This decrease may have caused a crisis for the simple microorganisms of the time, ultimately leading to the evolution of complex cells.
The oldest known living organisms were single-celled microorganisms called bacteria and archaea. However, nowadays there is a third group of organisms called eukaryotes, which have larger and more complex cells. Many eukaryotes are single-celled, but all multicellular plants and animals, including humans, also fall into this group.
The first eukaryotes are thought to have evolved between 1.8 and 1.2 billion years ago, but it isn鈥檛 clear why. Trace metals may be the key, say Indrani Mukherjee and Ross Large at the University of Tasmania in Australia.
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All living cells depend on small amounts of metals like iron, says Mukherjee. For instance, trace metals are used in the enzymes that power metabolism. 鈥淭race elements form structural components of these enzymes that speed up the reactions that actually sustain life,鈥� she says.
Mukherjee and Large have traced how the abundances of 12 trace metals varied between 3.6 billion years ago and today, by combining 1500 new analyses with 4147 previous ones. Past levels of trace metals were recorded in pyrite, a mineral that forms in the sea and which traps a range of metals.
Levels of metals like iron, nickel and copper were consistently high from 3.6 to 2 billion years ago, but then dramatically fell and stayed low until 1.2 billion years ago.
鈥淭his was a stressful time for the organisms,鈥� says Mukherjee.
To stay alive, Mukherjee suggests, microorganisms started taking other microbes inside themselves 鈥� either to store them for later digestion, or as cooperative partners. Some of the key internal structures of eukaryotic cells are known to be descended from bacteria that were once free-living.
Evolution probably wasn鈥檛 specifically favouring increased complexity, says Mukherjee. 鈥淚t was more like, this is a stressful time, how can we cope, and then that [increase in complexity] was another step that just happened, owing to the ingestion of these particles.鈥�
The root cause of the trace metal shortage was a shift in the kinds of rocks that were being built into continents, and which were ultimately eroded into the ocean, says Mukherjee.
Geology
