We know little about Earth's surface temperatures for the first 4 billion years or so of its history. This presents a limitation into research of life's origins on Earth and how it might arise on distant worlds.
Now researchers suggest that by resurrecting ancient enzymes they could estimate the temperatures in which these organisms likely evolved billions of years ago. The scientists recently published their findings in the journal Proceedings of the National Academy of Sciences.
"We need a better understanding of not only how life first evolved on Earth, but how life and the Earth's environment co-evolved over billions of years of geological history," said lead author Amanda Garcia, a paleogeobiologist at the University of California, Los Angeles. "A similar co-evolution seems certain to be the case for any life elsewhere in the Universe." [Ancient Earth: Pummeled, Cracked and Oozing Magma (Visualization)]
Garcia and her colleagues focused on the history of Earth's surface temperatures. Rocks offer many clues to deduce temperatures over the last 550 million years in the Phanerozoic Era, when complex, multicellular life took off, including that of humans. However, few such "paleo-thermometers" exist for the earlier Precambrian Era, spanning the Earth's formation 4.6 billion years ago and the rise of life.
Earlier geological evidence has suggested that 3.5 billion years ago, during the Archean Eon, the oceans were 131 degrees to 185 degrees F (55 degrees to 85 degrees C). They cooled dramatically to current average temperatures of 59 degrees F (15 degrees C). Scientists made these estimates by examining oxygen and silicon isotopes in marine rocks. Quartz-rich rocks in the seabed, known as cherts, have higher levels of the heavier oxygen-18 and silicon-30 isotopes as the seawater gets colder. In principle, the ratio of heavier to lighter oxygen and silicon isotopes can shed light on ancient temperatures.
But such paleo-thermometers do not adequately take into account how these rocks or the ocean might have changed over the course of billions of years. Perhaps the isotopic ratios in seawater varied over time in response to physical or chemical alterations, such as water flows off the land or from hydrothermal vents.