Australasian Science: Australia's authority on science since 1938

Fool’s Gold & the Ascent of Man

Members of the team sampling 480-million-year-old black shales from the Meguma Terrain in Nova Scotia, Canada.

Members of the team sampling 480-million-year-old black shales from the Meguma Terrain in Nova Scotia, Canada.

By Ross Large

Ancient samples of pyrite, or fool’s gold, have revealed the role of plate tectonics in bursts of evolution and mass extinction events. Did man ultimately originate from mega-mountains?

As a young scientist I often dreamt about going back in time to observe the surge and decline of life that makes up Darwin’s evolutionary theory. Imagine the excitement of riding a time machine that takes you back to when the Earth was 3.6 billion years old, and then progressing forward every 100 million years, making 36 stops along the way to observe changes to the Earth and the march of evolution: when life first appeared; the “boring billion” years when life stagnated; the Cambrian explosion that kick-started complex life; the five great mass extinction events; and the final appearance of man.

Now I have been fortunate enough to achieve this dream. A team I have led at The University of Tasmania has developed such a “time machine” by measuring more than 5000 crystals of pyrite using a laser beam attached to a super-sensitive chemical analyser. The results have led to a revolutionary change in our understanding of the symbiotic relationship between geological and evolutionary processes on Earth.

The pyrite crystals we analysed recorded changes in the chemistry of ancient oceans, enabling us to determine how the evolution of bacteria and higher life forms was affected, or even controlled, by concentrations of ocean trace elements.

The analytical data showed that the earliest oceans, more than 2 billion years ago, contained much higher levels of nickel, cobalt, iron, arsenic and gold than modern oceans. By studying pyrite textures in the sedimentary rocks we were able to observe the beginnings of life in the oceans. The evolution of the first prokaryotic bacteria depended on high levels of nickel and cobalt at this time.

Over the next billion years, known among geologists as the “boring billion”, our time machine indicated that the chemistry of the oceans became more alkaline. Many trace elements critical for life dropped to very low levels. At this time, life slowed to a virtual standstill, and other metals such as zinc and copper replaced nickel and cobalt as the controls on evolutionary pathways.

Between time machine landings at 600 million and 500 million years ago, we observed a dramatic change in the oceans. Trace elements critical for life, particularly molybdenum, selenium, nickel and manganese, increased dramatically in the ocean, creating a massive increase in bacterial blooms leading to the Cambrian explosion of life. This event was accompanied by a dynamic flux in oxygen in the atmosphere to levels similar to those required to sustain complex life.

The next five landings (500 million years to the present) turned out to be the most exciting for our team, with such rapid evolutionary change that extra landings were required to enable us to join the dots.

Connecting the Dots of Life

Our dataset of more than 3500 pyrite crystal laser analyses for the past 500 million years showed that the nutrient elements critical for life – selenium, molybdenum and cadmium – had a cyclic pattern of variation. This suggested that high concentrations of nutrient elements in the oceans were ideal for life and evolutionary change, while evolutionary stagnation was likely at low levels. Thus we could explain why bursts of evolution were interspersed with periods of little evolution. We are currently in a period of high nutrient levels, ideal for life in the oceans.

However, what totally surprised us was that three of the periods of extremely low nutrient elements corresponded precisely with three of the five major mass extinction events in the oceans.

Based on this result we determined to set our time machine precisely on these three mass extinction events at 450 million years ago, 375 million years ago and 200 million years ago, by analysing hundreds of pyrite crystals of these three ages.

The Chemical Window of Life

At this stage of the research we needed to match the results of geology and ocean chemistry with palaeontology, evolutionary biology and toxicology to solve the next big question: “What precisely caused the mass extinction events?”

I invited Prof John Long of Flinders University to become involved, and he formed a new group of international scientists that joined our team and targeted the element selenium as the devil in the detail. This group unearthed some important facts about selenium.

First, selenium is a unique element, with a critically small concentration window for life: too much is toxic, but too little cannot sustain life. Selenium is also strongly held in the structure of pyrite crystals, and can be accurately identified by our laser time machine.

Second, selenium is required by nearly all species to promote life. In fact, humans are dependent on minute traces of selenium. Disease in humans and animals is common in parts of the world where the soils are either overenriched or deficient in selenium.

Third, a survey of human blood concentrations of selenium in healthy adults from 69 countries found that nutritional selenium deficiency is highly prevalent in 21 countries and moderately prevalent in 16 countries. This suggests that many people do not consume enough selenium to support selenium-dependent enzymes that are required for optimum brain functions. The scientific data indicates that selenium concentration is too low in food produced in most of Europe, parts of Africa, Asia and New Zealand. It has been estimated that the number of selenium-deficient people in the world is in the range of 500–1000 million.

When we combined these studies with our pyrite data it became obvious that short periods of extreme selenium deficiency in the ancient oceans was the most likely cause of the three major global mass extinction events, hundreds of millions of years ago.

  1. The Late Ordovician event 445 million years ago wiped out 85% of all species in the oceans, including nautiloids, brachiopods and corals.
  2. The Late Devonian event 375 million years ago wiped out 80% of species, including the giant armoured placoderm fish.
  3. The Triassic/Jurassic boundary, 200 million years ago, wiped out 75% of species, including large amphibians and giant reptiles.
    1. Our data indicated that the other two major mass extinctions at the Permian/Triassic boundary and Cretaceous/Tertiary boundary were not due to selenium deficiency. Their cause is generally accepted to be due to megavolcanic eruptions and a giant meteorite impact, respectively.

      Did Man Evolve from the Mountains?

      A major feature of the Earth’s surface that would be obvious to time travellers stopping every 100 million years is the slow rearrangement of the continents due to plate tectonics.

      I was aware of a theory previously proposed by a team of scientists from the Australian National University that the Cambrian explosion of life 540 million years ago was ultimately driven by the collision of supercontinents during plate tectonics. As the theory goes, nutrients critical for life in the oceans principally come from the weathering and erosion of rocks on the continents. Weathering breaks down the minerals in the rocks and releases nutrient trace elements, which are the key to life and evolutionary change.

      This theory provided a link to our results. As erosion rates of mountains increase for extended periods, more life-supporting elements, including selenium, are supplied to the oceans. In the long term of geological history, erosion rates rise dramatically, enriching the oceans in nutrients during mountain-building events, and these major events are caused by the collision of tectonic plates.

      However as erosion reduces the scale of mountains, the supply of nutrients to the oceans declines and a period of nutrient-poor oceans leads to stagnant evolution and ultimately mass extinction. Our research defined five cycles of nutrient-rich oceans followed by nutrient–poor oceans (Fig. 1).

      Geologists have known since the 1960s that collisions of tectonic plates (called orogenic events) lead to the formation of continent-scale mountain ranges. Our research team has provided, for the first time, strong evidence to link plate tectonics, nutrient content in oceans, evolutionary patterns of life and mass extinction events. Our pyrite data shows that within the past 600 million years, the periods of nutrient-rich oceans correspond with global mountain-building events that coincide with major evolutionary activity.

      The peak of the first nutrient-rich ocean around 555–510 Ma corresponds with the East African–Antarctic orogenesis event, which produced a mountain range greater than 8000 km long and coincided with the onset and peak of the Cambrian explosion of life.

      The peak in the second nutrient cycle around 410–380 Ma in part corresponds with the termination of the main phase of orogenesis in the Appalachian–Caledonian system and with the first appearance of giant fish and very large terrestrial animals.

      The third nutrient cycle peak at 330–310 Ma corresponds with the early Pangaea assembly, the Appalachian (Hercynian) orogenesis event, and with a period of maximum forest expansion.

      The next nutrient peak at 280–260 Ma relates to the main phase of Pangaea assembly, including widespread mountain building in marginal regions (e.g. the Central Asian orogenic belt and Terra Australis Orogen) and diversification of terrestrial amphibians and conifers.

      The fifth peak around 150 Ma in the Late Jurassic corresponds with the Alpine orogeny and the appearance of birds.

      The final nutrient-rich ocean is at the present day, corresponding to the start of the Amasia (also termed Neo-Pangaea) assembly and the appearance of Homo erectus.

      Taken in its entirety, our new theory provides a giant twist to Darwin’s theory, and places biological evolution in a geological context: geological processes on Earth have been the driver of biological evolution. This is an exciting possibility that obviously requires further research before we can expect it to be fully accepted by other scientists. However, key questions are now apparent. Did humans evolve from the mountains? Will complex life only be found on other planets with evidence of plate tectonic and mountain-building processes?

      Ross Large is Distinguished Professor of Economic Geology at The University of Tasmania. The research described here was awarded the 2016 UNSW Eureka Prize for Excellence in Interdisciplinary Research.