SiS is proud to feature the winners of the "2008 Integrated Graduate Program in the Life Sciences (IGP) Science and Society Class Distinction Award." Written as part of a course on science and society, these papers were chosen by IGP faculty to be published on SiS. This month, we present the following piece by PhD student Sonali Joshi.
How life originated on earth is a question that people have pondered for ages. Theories abound, from those based on religious doctrine, to the purely scientific, to others that border on science fiction. One possibility that hovers on this border is the panspermia theory, which suggests that life on Earth did not originate on our planet, but was transported here from somewhere else in the universe. While this idea may seem straight out of a science fiction novel, some evidence suggests that an extraterrestrial origin of life may not be such a far out idea.
One argument that supports the panspermia theory is the emergence of life soon after the heavy bombardment period of earth, between 4 and 3.8 billion years ago. During this period, researchers believe the Earth endured an extended and very powerful series of meteor showers. However, the earliest evidence for life on Earth suggests it was present some 3.83 billion years ago, overlapping with this bombardment phase. These observations suggest that living things during this period would have faced extinction, contributing to the idea that life did not originate on Earth.
However, in order for life to originate elsewhere in the universe, there would have to be an environment on another planet capable of supporting it. Our study of the universe suggests that life as we know it would have a hard time surviving outside of the Earth. But, it is important to note that life on Earth can withstand many extreme conditions. Some bacteria grow at temperatures as high as 113°C. At the other end, microbes can thrive at temperatures as low as -18°C; many can be preserved in liquid nitrogen at -196°C. They can also tolerate high doses of ionizing and UV radiation, extreme pressure, etc. These observations suggest that it is difficult to define the conditions that favor life, and make it harder for us to predict that life is unique to Earth.
The presence of water elsewhere in the universe reinforces this. Mars is believed to have contained water in the past. Much excitement for the presence of life on Europa, one of Jupiter’s moons, has been fueled by speculations that it may have underground oceans. However, while water is essential for life that we are familiar with, its presence does not necessarily indicate the presence of life.
The fact that organic matter is relatively common in space could also support the idea of extraterrestrial life. Organic matter refers to matter composed of compounds that contain carbon. All living things on Earth are carbon-based. A variety of organic compounds have been detected in meteorites that have landed on earth, including amino acids, which are the building blocks of proteins (and proteins are primary components all of living cells). The presence of carbon-based matter in meteorites supports the possibility that life on our planet could have come from outer space. But, even though life on earth is composed of organic matter, organic matter itself is not considered life.
Even if extraterrestrial life did exist, proponents of the panspermia theory must still determine how life arrived on Earth. The best candidates to act as “seeds of life” are bacterial spores, which allow bacteria to remain in a dormant state in the absence of nutrients. Bacteria constitute about one-third of Earth’s biomass and are characterized by their ability to survive under extreme conditions—those that we initially believed were unable to support life. In light of panspermia, the important question is if bacteria or bacterial spores could survive in space.
To address this question, scientists at the German Aerospace Centre in Cologne designed experiments using the Russian FOTON satellite. They mixed bacterial spores with particles of clay, red sandstone, Martian meteorite or simulated Martian soil to make small lumps a centimeter across. The lumps were then exposed via the satellite to outer space. After two weeks of exposure, researchers found that nearly all of the bacterial spores mixed with red sandstone were able to survive. Another study showed that bacterial spores could survive the extreme conditions of outer space for six years if they were protected from extraterrestrial solar UV radiation. This would be possible if the spores traveled within comets or meteorites.
However, interplanetary distances are large, so the time a bacterial spore would have to spend in a meteorite or comet before hitting a host planet could range in the millions of years. Two studies involving the isolation of bacterial spores, either from the abdomen of extinct bees preserved in amber or from a brine inclusion in an old salt crystal from the Permian Salado formation, suggest that bacterial spores can remain viable for up to 250 million years. Thus, bacterial spores could potentially account for life on earth.
But are there bacterial spores floating through space? One study focused on the heat radiation emitted from Halley’s Comet's dust particles as the comet approached the sun. The particles' radiation fingerprint corresponded surprisingly well to that of bacteria heated to elevated temperatures – no material other than bacteria matched the observed spectrum. As comets are known to have collided with Earth at different points in the past, this observation presents an interesting argument for panspermia. While this study does not provide conclusive evidence for presence of life in outer space, it does raise the possibility that our galaxy may be littered with bacterial spores.
An important thing to note about the panspermia hypothesis is that it gives no explanation for how life that arrived on Earth came to be. Even if we are able to show that life on Earth was a result of panspermia, the question of where and how life originated will be a lot harder to answer. So far our knowledge of the solar system suggests that life is unique to Earth, but, as science and technology advance, we will have to modify ideas that we currently regard as facts. It remains to be seen if the questions regarding the origin of life on Earth and the origin of life in the universe have the same answer.