Habitability and Biology - page 4
When Did Life First Arise on Earth?Modern humans have walked the Earth for only a very small portion of our planet's history - about 100,000 of its 4.6 billion years. But there is evidence to suggest that life has been around on Earth for quite a long time. For example, ancient rocks called stromatolites found in various regions of the Earth have a layered structure that is nearly identical to that of microbial mats formed today by large colonies of microbes. These mats grow as sediments get deposited on top of them, which forces the microbes to move upward in order to remain close enough to the surface to use photosynthesis. If ancient stromatolites formed the same way that microbial mats do today, then photosynthetic life could have already evolved significantly by 3.5 billion years ago.
This stromatolite fossil was found in late pre-Cambrian rocks in Montana. The flat layers are fossilized microbial mats, and the curved layers are fossilized mounds like those found on Earth today. (Image credit: Allan Treiman, Lunar and Planetary Institute; and Museum of the Rockies, Bozeman, Montana)
Some biologists believe that life began in freshwater ponds, while other evidence indicates that the first organisms may have gotten their start in deep-sea thermal vents. One reason for this is that early Earth experienced a far greater number of impacts than it does today, and the deep ocean would have provided a safe haven; whereas surface ponds would have left organisms exposed to these catastrophic events. One such impact event occurred about 65 million years ago when an asteroid hit the Earth near what is now the Yucatan region of Mexico. The impact, known as the Cretaceous-Tertiary Event (K-T) destroyed about 70% of all living species on Earth, including the dinosaurs.
This image shows an artist's rendering of the Cretaceous-Tertiary impact event, which occurred about 65 million years ago near what is now the Yucatan Peninsula in Mexico. The impact killed about 70% of the species on Earth. (Image credit: NASA)
The Fermi Paradox: Are We Alone?Our solar system is only one of more than 100 billion star systems in the Milky Way Galaxy. Based on these sheer numbers and the age of the universe, it stands to reason that extraterrestrial life should be common in our galaxy. This idea is supported by estimates based on the Drake Equation.
Large radio telescopes, which analyze radio waves from distant stars and galaxies, are used in the search for extraterrestrial life. (Image credit: NASA)
In the early 1950s, during a lunch discussion of the premise that our galaxy should have been colonized by now, Nobel prize-winning physicist Enrico Fermi is reported to have asked the question, "So where is everybody?" Fermi's simple question formed the basis for the Fermi Paradox, which states:
The belief that the universe contains many technologically advanced civilizations, combined with our lack of observational evidence to support that view, is inconsistent.
The first and most obvious way to resolve the Paradox is to find evidence of extraterrestrial intelligence. Since the 1960s, the Search for Extraterrestrial Intelligence (SETI) has involved actively listening for signals from other regions of our galaxy, but so far these searches have not turned up any conclusive evidence.
There are other possible resolutions to the Fermi Paradox: