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Mountains Drove Bursts of Evolution and Extinction

The oldest life on Earth is still going strong today. Stromatolites are communities made up largely of blue-green algae. This image shows a living colony of stromatolites at Shark Bay, Western Australia. Credit: Ken McNamara

The oldest life on Earth is still going strong today. Stromatolites are communities made up largely of blue-green algae. This image shows a living colony of stromatolites at Shark Bay, Western Australia. Credit: Ken McNamara

By Ross Large, John Long & Indrani Mukherjee

Bursts of evolution and mass extinction events coincide with mountain-building events that have influenced nutrient levels in the oceans.

The oceans became the cradle of life almost 4 billion years ago, and have been home to all life on Earth for roughly 90% of the time since life first appeared. Prior to about 470 million years ago, all life on Earth lived in the oceans.

We can trace single-celled organisms back to about 3.54 billion years ago and complex organisms (eukaryotic cells with a nucleus) to 2 billion years ago. Complex multicellular animals (metazoans) appeared at least 580 million years ago, but the most significant evolutionary event leading to all modern animal phyla occurred during the Cambrian explosion 550–520 million years ago. Our back-boned fishy ancestors only invaded land about 370 million years ago, enabling humans to ultimately evolve from fully terrestrial tetrapod ancestors.

Life in the oceans has always required nutrient trace elements to survive and evolve. These include iron, phosphorus, nickel, cobalt, molybdenum and selenium. These and other “essential trace elements” were so named because they are critical for all life on this planet to survive. Periods in the Earth’s history when these nutrients were low in the oceans therefore caused stress for organisms, and in extreme cases mass extinctions followed (AustSci, May 2014, pp.22–25, When nutrients were abundant, life radiated to the far corners of the oceans.

Our analysis of the trace element content of ancient marine pyrites has found that the nutrient trace element concentrations in the ancient oceans have followed a cyclic pattern. In the oldest oceans during the Precambrian 3600–540 million years ago, the cycles have a frequency of about 500 million years, whereas in the younger Phanerozoic oceans (since 542 million years ago) the frequency drops to 70–100 million years (Fig. 1). These cycles of nutrient abundance are vitally important as they appear to be the driver for evolutionary change and the cause of mass extinction events. Each cycle starts with a rise in nutrients, to a maximum, followed by a rapid fall ending in a mass extinction event.

We set out to determine the cause of the nutrient cycles and the reasons for their change in frequency. Previous scientific research has shown that the source of nutrients is from the erosion of rocks on the continents, with rivers transporting nutrients to the oceans. A secondary source are volcanic vents called “black smokers” on the deep ocean floor. These may have been important during particular intervals when smokers were especially active, churning out large volumes of trace elements into the oceans.

Weathering and erosion delivers more nutrients in active tectonic areas, where tectonic plates collide to instigate mountain-building phases called orogenesis. This led our research team to propose that a linked sequence of events led to the nutrient cycles that were so important for evolution in the oceans.

Not only do the increased nutrients lead to increased life in the oceans (as measured by increased biomass “productivity”), but also to the release of oxygen to the atmosphere, thus promoting a slow global rise in atmospheric oxygen.

There has been much debate in the scientific literature concerning oxygen and life, and which came first. It’s a bit like the horse and the cart. Did oxygen rise dramatically around 540 million years ago and lead to the Cambrian explosion of life? Or did evolutionary change rise dramatically around 540 million years ago, leading to a massive change in the amount of oxygen released to the atmosphere?

Our research suggests it is more likely to be the latter. However, we also consider it is the rapid rise in nutrients, around 550 million years ago, caused by a phase of mountain-building, that led to rapid evolutionary change and the subsequent build-up of atmospheric oxygen.

This theory is supported by the coincidence of the peaks of nutrient cycles with major mountain-building events around the globe over the past 550 million years. Figure 2 shows that the major evolutionary steps in the Phanerozoic correspond with peaks in the nutrient cycles.

An interesting and important conclusion that we can draw from our research is that all major Earth and life processes through time have a cyclic nature. On timescales of tens to hundreds of millions of years, nothing seems to be flatlining or static. Plate collisions are cyclic, orogenesis is cyclic, ocean nutrient concentrations are cyclic, life in the oceans is cyclic (each cycle starting with radiation and ending with mass extinction), oxygen in the atmosphere is cyclic, and no doubt carbon dioxide in the atmosphere will turn out to be cyclic.

Ross Large is Emeritus Distinguished Professor of Geology at the University of Tasmania. John Long is Strategic Professor in Palaeontology at Flinders University. Indrani Mukherjee is a Postdoctoral Fellow in Geology at the University of Tasmania.