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Fool’s Gold: The Search for Early Life

Michaela Partridge examining a banded iron formation from the Archean.

Michaela Partridge in the Pilbara examining a banded iron formation from the Archean.

By Michaela Partridge

The golden mineral, pyrite, is a valuable tool in the search for the secrets of early life on Earth.

Imagine a planet where the atmosphere is a hazy methane greenhouse and the surface is barely warmed by a faint young star that is only two-thirds as bright as the Sun we know today. An alien world with anoxic oceans full of microscopic life forms, most of which would perish in the oxygen-rich atmosphere that we depend on for our survival.

This planet is not separated from ours by the vastness of space, but by time. It is the Archean Earth. The Archean represents the first two billion or so years in Earth’s history, up until we start to see evidence of widespread atmospheric oxygen in the rock record 2.5 billion years ago. The composition of Earth’s modern atmosphere is a hot topic these days, but how do we learn about what the atmosphere was like in Earth’s deep past?

In the ancient rock record, all the available information about the complexity and diversity of the biosphere, the oceans and the atmosphere – from above the clouds to below the sea floor – is all squashed into one big rock pancake, so trying to fully reconstruct the early oceans and atmosphere is a tricky business. There are few preserved remnants of very ancient organisms so we have little direct evidence of these early life forms, but we can see the effect that ancient organisms had on their habitats.

We can study this effect on the chemistry of Archean rocks and minerals to determine not just when and where particular organisms existed in Earth’s history, but also what sort of environments they lived in. Some of the best rocks to study for evidence of early life on Earth are found in Western Australia’s Hamersley Basin.

A valuable tool that can be used to decipher some of the secrets in these ancient rocks is a mineral that the untrained eye may well mistake for a very different kind of treasure. Rocks containing the golden metallic mineral pyrite – commonly known as “fool’s gold” – can be used to identify diverse microbial habitats on early Earth.

Grains of this sulfur-bearing mineral found in two- to three-billion-year-old rocks can come from a variety of different sources. Some pyrite in ancient rocks was produced by the activity of microscopic organisms that lived in the oceans billions of years ago. These types of microbes feed on sulfur and, given the right conditions, will essentially create pyrite as a waste product. Careful analysis of pyrite grains and the ancient rocks they are found in can reveal if any organisms were present when the rocks formed, and whether these organisms lived in a high or low oxygen environment.

The life forms that thrived on Earth billions of years ago were all microscopic organisms with very simple metabolisms. They subsisted on a diet of chemical cocktails, taking what they needed and producing a different chemical product as waste. Other organisms could then use that “waste” as their nutrient source.

Carbon and sulfur are popular menu items for simple microbes. Because of this, the effects of some metabolic processes can be preserved in sulfur-bearing minerals and carbon-rich rocks for billions of years.

There may have been many complex microbial communities cycling elements around the biosphere in the early years of life on Earth. This makes things rather complicated, because elements like sulfur that end up being preserved in the rock record as minerals such as pyrite may have already been cycled through the metabolisms of several different organisms. The aim of my research at the University of Queensland is to decipher the remnants of this recycled chemical buffet.

Most animals have preferences for certain types of food, and microscopic organisms are no different. While they may have highly specialised diets of very specific elements, they often have preferences for certain isotopes of those elements. Isotopes are simply different forms of the same element with very slightly different masses.

Generally organisms will consume lighter isotopes if they have a choice, as this lighter option is often easier to metabolise. This means that the distributions of isotopes of appetising elements like carbon and sulfur can help us track changes in microbial abundances and habitats over geological time. Microbes living in an oxygen-poor habitat will leave behind a very different chemical footprint to those from an environment containing oxygen. In this way isotopes are like chemical fossils that can be used to determine not just when and where particular organisms were active, but also what sort of environments they came from.

One of the reasons Archean sulfur-bearing minerals like pyrite are so useful is that in a very low-oxygen atmosphere, such as on early Earth, sulfur isotopes are fractionated, or separated, differently to the way they are in our modern atmosphere. This allows us to trace where some of the sulfur in the rock record originally came from.

Before Earth had developed its protective ozone layer about 2.4 billion years ago, sulfur dioxide in volcanic gases was subjected to deep ultraviolet radiation, which converted the gas to oxygen-bearing sulfate and oxygen-free sulfur, and also seems to have affected the distribution of sulfur isotopes in these products. We can measure this phenomenon to determine whether sulfur isotopes started out in oxygen-free or oxygen-bearing sulfur compounds. This, in turn, tells us a great deal about the environments and metabolisms that this sulfur may have passed through before it ended up in the rock record as part of a tiny golden speck of pyrite.

In some Archean rocks, a pyrite grain containing sulfur from a particular source will be composed of specific sulfur isotopes. This is important because different sulfur sources can only remain separate under low oxygen conditions.

My research group is using this information to develop a three-dimensional model of Earth’s early atmosphere and oceans. We do this by establishing the origins of individual components of ancient rocks from the Hamersley Basin.

We will look at one grain of pyrite in a particular rock and determine that a certain process in a specific environment formed that grain. We can then look at the other parts of that rock and decide whether it formed in the same environment or from somewhere above or below our first pyrite grain.

Even though they may have initially come from quite different environmental layers, everything settled in the same place on the seafloor and that is why we find all of these different components together in the same rock. It is a painstaking process, but it provides a wonderfully detailed snapshot of our planet’s early years.

Through detailed study of ancient rocks and minerals, it appears that diverse microbial life was already established

3.8 billion years ago, which is only a few hundred million years after liquid water first appeared on Earth’s surface. After that, not much changed for about a billion years. That’s a billion years with little evolutionary change. However, at a critical point in Earth’s past, two-and-a-half billion years ago, tectonic activity greatly increased the habitat area of small communities of microbes that produced oxygen, and this caused a population explosion of this previously uncommon form of life.

Before this transition event, Earth’s atmosphere contained very little oxygen, and was instead dominated by methane, which is a very strong greenhouse gas. The greenhouse effect this created was important because back then the Sun was only about two-thirds as bright as it is today. Anaerobic organisms that could only survive in low oxygen conditions produced this methane.

The population explosion of oxygen-producing microbes caused an increase of oxygen in the atmosphere and drove anaerobic organisms from habitats they had quietly occupied for more than a billion years. The first of these oxygen-producing microbes were probably also anaerobic, so the oxygen-rich environment they created was toxic to them, but organisms soon developed that flourished in Earth’s new oxygen-rich habitats. Earth’s atmosphere and biosphere were transformed forever.

Microscopic organisms still produce most of the oxygen in our atmosphere today, so mapping when and where different types of microbes existed allows us to trace the spread of oxygen from low concentrations in localised environments three billion years ago to the oxygen-rich planet we know today.

This understanding of the early evolution of life on Earth, obtained from minerals like pyrite, will help to improve what we know about the sort of life and atmospheres that might evolve on other planets and moons, both in our own solar system and beyond. While there is a very good chance that there is life on other planets and moons, it might be rare to have a planet with lots oxygen in the atmosphere and as many different kinds of life as we have here on Earth.

Two-and-a-half billion years ago, a key life form on Earth destroyed previously stable ecosystems and changed our planet forever. This transformation worked out quite well for us oxygen fans. However, we might not be as lucky if some of Earth’s inhabitants radically change our atmosphere again.

Michaela Partridge is completing her PhD in astrobiology at the University of Queensland.