A Question of Climate
By Leslie Mullen
Our planet was born hot. Earth formed around 4.5 billion years ago by rocks bashing together so violently they became molten and stuck together. By about 4.3 billion years ago, once Earth was no longer being continuously pummeled by huge rocks from outer space, the magma had cooled and continental crust had begun to form. Water vapor rained out of the atmosphere and created oceans on the young surface. Life may have originated not too long after this.
The hot nature of our planet’s earliest days seems to be reflected in the tree of life – the most ancient branches are thermophiles, microbes that thrive in temperatures of 50 degrees C (122 F) or warmer. These single-celled life forms can be found today living in volcanic vents under the sea, or in steaming geysers like those in Yellowstone National Park.
But doubts remain about the temperature of the Earth during life’s origin, and whether the thermophiles actually represent the first life to appear on our planet.
To peer into the past, scientists look to the most ancient rocks. Rocks can tell us what sort of gases made up the atmosphere, and the kind of chemical interactions that were taking place in the environment. Unfortunately most of the rocks on our planet have been irrevocably altered, their history erased. The tectonic plates that make up the planet’s crust bash into each other to build mountains and dive underneath one another to become molten once again.
However, some rocks have opted out of this rock recycling and burial program due to the random luck of geography. Greenland has the most ancient sedimentary rocks, dating back to about 3.8 billion years ago. Rocks dating back to 3.75 billion years ago recently have been found in Canada, while Australia and South Africa have 3.5 billion year old rocks. (The oldest rocks are 4 billion-year-old volcanic gneisses from northwest Canada, but rocks that formed beneath a volcano can’t tell us as much about the surface environment. Sedimentary rocks are made from the slow accumulation of layers of soil, and this, along with the fossils that are trapped inside these layers, provide a better picture of what the environment was like).
Scientists are studying these rocks to better understand how and when Earth started to cool. However, climate is a complex phenomenon and many factors can affect it. Scientists looking at different aspects within the rock record often end up disagreeing about the past.
The sun was a faint young star around 4 billion years ago, so Earth would have received less solar radiation. A planet’s temperature depends on more than the star it orbits, however. Our planetary neighbor Venus has a surface temperature exceeding 400 degrees C (800 F). While Venus is closer to the sun than the Earth, the main reason for its hot climate comes from Venus’s thick greenhouse atmosphere that traps heat.
Theoretical models suggest that Earth’s first atmosphere was composed of greenhouse gases like carbon dioxide, methane, and water vapor, as well as hydrogen and nitrogen. This dense atmosphere would have kept the Earth scorching hot. After this point, our knowledge of the climactic changes that occurred is as hazy as the atmosphere itself.
Some scientists, like Norm Sleep of Stanford University, think the early Earth quickly became cold once tectonics kicked in. Carbonate minerals that formed due to the high levels of carbon dioxide in the water and atmosphere would have become buried, removing a great deal of carbon from circulation and resulting in an atmosphere with less carbon dioxide. Lower amounts of this greenhouse gas would have led to rapid cooling, until temperatures averaged a moderate 30 degrees C (86 F). In fact, Sleep thinks early CO2 levels were so low that the ancient Earth at various times became a snowball — so cold that the planet was almost entirely covered with a crust of ice.
Other scientists, like David Schwartzman of Howard University, think the early Earth remained hot. Schwartzman doesn’t think tectonics buried all the carbon. Instead, he says carbon dioxide remained a major climatic factor for a long time, keeping the Earth toasty up until 1.5 billion years ago, with temperatures averaging between 50 to 70 degrees C (122 to 158 F).
“There was at least one bar of carbon dioxide up to about 2.8 billion years ago,” says Schwartzman. “That’s 10,000 times the level we have now.”
Around that time, cyanobacteria and other microbial life forms began to proliferate and suck up huge amounts of carbon. Then methane, which was produced by some of this burgeoning microbial life, became a more dominant gas. The methane was a tenth or less abundant than CO2 back then, “but a little methane goes a long way, as we know from global warming today,” says Schwartzman. Methane continued to keep Earth warm until 2.3 billion years ago, but then a waste gas produced by cyanobacteria started to have a big impact. This waste gas was oxygen, and it had been slowly building over millions of years. The oxygen reacted with methane, and as methane levels began to plunge so too did temperatures.
Schwartzman says brief periods of cooling occurred within the overall trend of warmth. There was a glacial period around 2.9 billion years ago, and another at 2.3 billion years ago. Schwartzman credits both these glaciations to the rise of oxygen in the atmosphere. Temperatures rebounded after the first glaciation, only to again experience brief but steep drops in later years. The overall warm climate trend finally ended once free oxygen stopped finding elements to react with and atmospheric oxygen levels began to stabilize.
Jim Kasting of Penn State University has a different view. Rather than Schwartzman’s scenario of an extended hot period followed by a relatively recent cooling, or an early and immediate drop in temperature as in Sleep’s scenario, Kasting thinks the Earth’s cool-down was more gradual. He says the hot early Earth started to cool about 4 billion years ago, and, thanks to the carbon burial of tectonics, was cool enough by 2.9 billion years ago to develop glaciers.
Scientists debate about different rock record indications of past temperatures. They argue about the ratio of oxygen isotopes through geologic time, the burial rate of tectonics, the alteration of silica, the effect of biology on weathering rates, and various other complicated and often interconnected processes that can turn something as simple as a rock into a palimpsest of historical mystery.
For astrobiologists, the ultimate question about Earth’s early climate is: What temperature was needed for life to arise? Or, perhaps, can life come about in many different temperature regimes? Many scientists, including Schwartzman, think life on Earth formed in the ultrahot hydrothermal vents on the ocean floor. While the deeply rooted nature of thermophiles in the tree of life lends support to this, some scientists point out that the delicate machinery of life, such as proteins and DNA molecules, often break apart at high temperatures. Sleep, meanwhile, thinks life probably originated under cold conditions, where freeze-thaw cycles created the energetic disequilibrium needed. However, Kasting thinks life most likely formed at moderate temperatures, after Earth had received tons of complex organics from meteor and comet impacts.
Perhaps the answer to the question about Earth’s — and life’s — early temperatures will be found in life itself. In a report recently published in the journal Nature, scientists reconstructed proteins from ancient bacteria to measure the Earth’s temperature over the ages. By comparing the heat sensitivity of the reconstructed proteins, they found that life lived in a hot environment of 75 degrees C (165 F) 3.5 billion years ago, and this environment gradually cooled to 40 degrees C (100 F) by 500 million years ago.
“By studying proteins encoded by these primordial genes, we are able to infer information about the environmental conditions of the early Earth,” says Eric Gaucher, president of scientific research at the Foundation for Applied Molecular Evolution in Gainesville, Florida and the study’s lead scientist. “Genes evolve to adapt to the environmental conditions in which an organism lives. Resurrecting these since long-extinct genes gives us the opportunity to analyze and dissect the ancient surroundings that have been recorded in the gene sequence. The genes essentially behave as dynamic fossils.”
