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Geological and paleontological evidence as well as chemical experiments have allowed us to piece together a picture of how life on Earth evolved from the earliest single-celled organisms to the diversity of single- and multi-celled organisms that have lived through history.
Geological evidence helps provide a timeline for the origin of life on Earth. It is thought that Earth formed approximately 4.6 billion years ago (bya). The earliest fossil evidence for life has been dated to 3.5 bya. This helps give scientists an idea of when life originated. While the timeline of the history of life (Figure 7.08) has been laid out through fossil evidence, how exactly life originated is less certain.
Figure 7.08: History of Life on Earth
Models of the Origin of Life
There are several models and theories about how life may have formed from inorganic precursor molecules. One theory is that organic molecules were transported to Earth from space via a meteorite or other event. Another theory is that life originated near hydrothermal vents in the ocean that provide mineral rich, warm environments. One thing we do know is that there was a lack of atmospheric oxygen on early Earth. This could have created an environment that facilitated the synthesis of organic molecules. Chemical experiments have shown that it is possible for complex organic molecules to form from inorganic molecules without life already being present. As we have discussed in other units, different molecules serve as monomers for more complex molecules. Once these monomers formed on Earth, they then could have joined together to form the polymers that life now relies on to replicate, store, and transfer information.
RNA World Hypothesis
The RNA world hypothesis suggests that RNA could have been the earliest genetic material. Research has shown that RNA can both store genetic information and catalyze chemical reactions, as it does in ribosomes. This ability of RNA to perform two functions that are essential to life as we know it suggests it could have been an early molecule on Earth that helped produce life. RNA replication also relies on base-pairing to transfer genetic information. This supports that RNA could have been the original mechanism of sharing and passing on genetic information.
