|The First Life on Earth
Earth was able to support life only after the planet had cooled enough for a rocky crust to solidify. Once that happened,
water vapor from volcanoes condensed in the atmosphere, fell as rain, and collected on the Earth’s surface. Besides water vapor, volcanoes also produced
gases rich in the basic ingredients of life: carbon, hydrogen, oxygen, and nitrogen. Toxic gases such as ammonia and methane were common. At this point,
Earth's early atmosphere consisted entirely of these volcanic gases, and there was no free oxygen. In the primordial “soup” of the early seas,
organic molecules concentrated, formed more complex molecules, and became simple cells.
The transition from complex organic molecules to living cells could have occurred in several environments. Small, warm ponds are one possibility, but recent
work has suggested that deep-sea hydrothermal vents,
such as those found along mid-ocean spreading centers today, may have been the cradle of Earth's life. These environments contain the chemicals and the source
of energy needed to synthesize more complex organic structures. Although scientists have not succeeded in creating life from organic molecules in the laboratory,
they have reproduced many of the intermediate steps.
So what were the first living things and when did they appear? Studies of genetic material indicate that a living group of single-celled organisms called Archaea
may share many features with early life on Earth. Many Archaea now live in hot springs, deep-sea vents, saline water, and other harsh environments. If the first
organisms resembled modern Archaea, they also may have lived in such places, but direct evidence for early life is controversial because it is difficult to
distinguish between complex inorganic structures and simple biological ones in the geologic record. The oldest evidence for life may be 3.5-billion-year-old
sedimentary structures from Australia that resemble stromatolites.
Stromatolites are created today by living mats of microorganisms
(mostly cyanobacteria, or blue-green algae). These primitive organisms trap thin layers of sediment with their sticky filaments and grow upward to get light for
photosynthesis. Modern-day examples of stromatolites can be
found in waters off Australia, the Bahamas, and Belize.
In the Archean structures, layers similar to those seen in living stromatolites are evident, and secondary structures interpreted as simple filamentous microfossils
have been recovered from the layers. The biotic origin of the structures has, however, been questioned. Both the supposed Archean stromatolites and the microfossils
may have been produced by inorganic processes. Regardless, uncontested microfossils and chemical traces of life were present at least by 2.7 billion years ago.
Stromatolites that were produced by microorganisms are abundant later in the Archean and throughout the
Proterozoic. These sedimentary structures, formed by organic
processes, provide important evidence of early life. At present, we can say with certainty that life had evolved by 2.7 billion years ago, and possibly as early as
3.5 billion years ago.
Eon Overview |
Early Continents and Oceans |
The First Life on Earth
Changes in the Atmosphere
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