Chapter 11: A Biography of Earth
Feature Articles
1: The Moon: A Source of Insight into the Hadean Earth?
2: The Cradle of Life
3: Clues in the Fossil Record
The Cradle of Life
by Stephen Marshak
Black Smoker
Credit:NOAA
Most questions concerning how life began on Earth remain unanswered today. But geological studies have provided some important insight into the process. Life forms consist of specialized organic chemicals organized into cells. Laboratory experiments show that amino acids, the building blocks of these chemicals, can be synthesized out of inorganic chemicals from elements that naturally exist at the surface of the Earth. But where did this process take place? Some scientists speculate that amino acids were created when lightning struck shallow seawater ponds that contained the appropriate raw materials. But even in the simplest amino acids cannot survive long in the presence of oxygen, for they will react with oxygen and fall apart. We now know that although the Archean atmosphere held very little oxygen, it was not totally oxygen-free-water molecules hit by ultraviolet radiation in the atmosphere broke down and supplied a trace of oxygen. Thus, amino acids could not have formed in warm ponds at the surface of the Earth.
In the 1970s, when geologists began using submarines to study Earth's mid-ocean ridge system, they found hot-water vents (black smokers) along the ridge axis (see Chapter 4). Bacteria have colonized the oxygen-free water around these vents. These organisms draw their life-giving energy from the chemical bonds in sulfide minerals that spew out of the vents. Thus, their metabolism differs fundamentally from that of familiar animals and plants, who draw energy from organic chemicals like sugar and starch: the burning of sugar and starch requires oxygen, while the burning of sulfide minerals does not. Considering this discovery, many geologists now speculate that the chemical components of life first formed somewhere in the hot water along the tens of thousands of kilometers of Archean mid-ocean ridges. The first living organisms were likely sulfide-eating bacteria.
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