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.

Stromatolite
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.

Chicxulub
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)
Although scientists still can't pinpoint exactly where or when life began on Earth, they do know that microbes were around long before plants and animals. In fact, for about 90% of Earth's history, only microscopic organisms inhabited the Earth. Given their extremely long record of survival on Earth, microbes could be the most likely signs of life we find on Mars.
  • The Biosphere

    • 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.

    Telescope
    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:

    1. Perhaps we really are alone in the galaxy. The conditions required for life to evolve are extremely rare or even unique to the Earth.
    2. A hazardous universe destroys intelligent life. Perhaps life does indeed arise elsewhere in our galaxy, but events such as ice ages and asteroid impacts prevent life from evolving.
    3. Intelligent life eventually destroys itself. It is the nature of technological civilizations to destroy themselves shortly after developing technology. Perhaps even humans will not exist long enough to detect other intelligent life.
    4. Communication with other life is impossible for technical reasons. If intelligent life exists in our galaxy, a physical separation of several thousand light years may mean that we are too far apart to communicate. We may also be too far apart in time; the odds of two intelligent civilizations existing at the same time may be very low. Other technical reasons relate to the technology itself - quite simply, we may be sending signals using the wrong radio frequencies. In addition, the communication technologically of an advanced civilization may be so beyond our own that we could not recognize a signal even if it came to us.
    5. There is intelligent life, but they have chosen not to communicate with us. Other civilizations may wish to avoid detection, or a civilization that is millions of years ahead of us may simply have no interest in Earth.
    Although there are a number of ways to potentially solve the Fermi Paradox, we currently have no way of knowing if any of these speculations are correct.
  • The Habitable Zone

    • Printable Version

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