16:29 14 November 2008 by Marcus Chown and Nov 13th 2008 The Economist print edition
EVOLUTION has come a long way since Charles Darwin’s time. Today it is not only animals and plants that are seen as having evolved over time, but also things that involve the hand of humans, like architecture, music, car design and even governments. Now rocks, too, seem to be showing evolutionary characteristics.
As life has evolved into all its abundance and diversity, Earth's rocks have come along for the ride. Two-thirds of the kingdom of minerals – the building blocks of rocks – can be traced to the emergence of life, say geologists.
This new perspective could shake up our picture of Earth's geological history, and might even help us find life on other planets.
"Rocks and life evolved in parallel," says Robert Hazen of the Carnegie Institution's Geophysical Laboratory in Washington DC. "It's so obvious - you wonder why we geologists didn't think of it before."
All rocks are made up of constituent minerals. For example, granite contains quartz, mica and feldspar. Geologists tend to classify such minerals in a methodical way, using factors such as their colour, hardness, and composition.
Hazen and his colleagues decided to take a fresh approach by exploring how the diversity, abundance and associations within the mineral kingdom have evolved over time. "What we have done differently is recognise that each mineral has its own story." he says.
Breath of life
According to Hazen, the story begins with a mere 12 minerals that existed in the dust swirling around in the clouds that would eventually form the solar system - minerals like diamond, created in the fury of supernova explosions. When the Sun ignited, the heat from this event boosted the number to around 60.
The formation of the Earth and subsequent geochemical processes upped that to around 500, and the switching on of the conveyor belt of plate tectonics to around 1500.
"But it was life, which made its first appearance about 4 billion years ago, that made the biggest difference," says Hazen. "It boosted the number of minerals to more than 4000."
Life brought profound changes to Earth's atmosphere and ocean chemistry.
Photosynthesising organisms created abundant atmospheric oxygen. Under this oxygen-rich environment, the chemical processes of oxidation and weathering generated a swathe of new species of metal-rich minerals, such as iron.
"Four billion years ago, metals on the surface like iron and copper remained pure and shiny," says Hazen. "But the new atmosphere oxidised them, creating a host of new minerals." Approximately half of the 4300 known mineral species are down to oxidation or weathering.
Microscopic algae, the earliest living organisms, drew carbon dioxide from the atmosphere and expelled oxygen. Over millions of years this created an oxygen-rich atmosphere that rapidly removed electrons from minerals near the surface, creating rust out of iron and forming thousands of new minerals from other metals like nickel, copper and uranium.
Living stones
Around two billion years later, the nature of the mineral kingdom evolved again with the emergence of hard-bodied marine organisms. Their bodies mineralised shells and skeletons for protection and support. Corals too started combining the calcium and carbonate that was floating freely in the water to construct reefs. All of the materials that animals made started to litter the sea floor and this vast accumulation of bone, shell and coral got pressed together into a mineral known as calcite.
On the land, plants produced acids around their roots that converted minerals of volcanic origin, like mica, feldspar and pyroxene, into clay minerals that ultimately formed intensely rich soils. This explains why volcanic islands like Hawaii are so lush.
Even the amounts of precious metals may have been influenced by life - microbes are thought to trigger the deposition of gold from hot waters inside rocks.
New minerals created by living things continue to turn up. One of the most recent discoveries was by Hexiong Yang, who named it Hazenite as a tribute to Dr Hazen, his former teacher. Hazenite is a mineral formed by microbes in the highly alkaline Mono Lake in California.
Search for ET
Hazen says the new perspective could aid the search for extraterrestrial life. He reckons probes sent to planets like Mars should be designed to look for particular mineral features such as oxidised zones in cliff faces.
"If we find certain minerals, they will point uniquely to certain organisms," he says.
With NASA’s Messenger probe now going into orbit around Mercury, Dr Hazen predicts that it will find only 300 or so minerals on the planet. If there are 500-1,000 detected, then it will suggest that there is a lot more to Mercury than anyone originally thought. And if minerals that depend upon life for their formation show up, then researchers will be flummoxed. The same is true for Mars and other planets—including the exoplanets that have been known about but which have just been seen for the first time orbiting stars outside the Solar System (see article). Dr Hazen argues that considering minerals in evolutionary terms is a powerful way to help identify how far a planet has developed geologically. Moreover it can tell you whether life was present at some point—and even whether it is present now.Gary Ernst at Stanford University describes the study as "breathtaking" and says the new perspective could revolutionise the way geologists describe the mineral kingdom.
Journal reference: American Mineralogist (vol 93, p 1693)
