Frontiers | Pages 160-161 | See Linked Version

Life on the Edge

As we explore our own planet, we find that life has taken root in even the most extreme environments. Earth is home to organisms that thrive in boiling liquids and grow without light deep in the ocean, where they are also subject to intense pressures. Our planet hosts a tiny bug that can endure conditions nearly as severe as those found in space, microbes in Antarctic ice, and moss that remained alive yet dormant while frozen for 40,000 years in the Siberian permafrost. These organisms redefine the notion of life-supporting environments.

A Tiny Survivor

The pinhead-sized tardigrade (left), which lives in moss and mud in roof gutters and the cracks of paving stones, can withstand pressures 6,000 times greater than at sea level and temperatures from near absolute zero to 250 degrees Fahrenheit. It also survives complete dehydration as well as laboratory exposure to a vacuum and to intense x-rays. Some tardigrades have been revived after lying dormant in dried moss in museums for more than 100 years.

Fountain of Life

More than a mile beneath the surface of the ocean, thermal vents (right) offer an unexpected oasis for creatures such as crustaceans, bivalves, and giant tube worms (above). These organisms are part of a geochemical ecosystem depending on bacteria that use hydrogen sulfide from vent water as their primary energy source. The water, superheated under pressure to as much as 700 degrees Fahrenheit, dissolves minerals from hot magma welling up through cracks in the ocean floor.

Cold Lovers

Colonies of organisms such as the cyanobacteria shown above exist beneath polar ice. Researchers liken these conditions to those on Mars. Dormant ancient microbes and even plants such as moss can remain preserved in ice, resuming metabolic activity after thousands to millions of years.

Heat Lovers

Organisms such as the bacterium Moorella obsidium (left) thrive in the near-boiling waters of geothermal hot springs in Yellowstone National Park. Gaining energy by oxidizing sulfur around the springs, such organisms produce sulfuric acid as waste.

out of the planet. Until recently, biologists didn't know that life could flourish in such extreme settings, often far beyond the reach of sunlight. These organisms--so-called extremophiles--have raised hopes that Earth's biological garden is just one of many oases in an otherwise barren cosmos.

Whether life has sprouted in other parts of the universe is one of the great cosmic questions we face today. We also seek to understand the behavior of matter on the tiniest scales and the origin and fate of the universe itself. Addressing these mysteries requires us to push our scientific skills to their limits. But if we continue to make steady progress in our explorations of motion, matter, and energy in the universe, even these answers may lie within reach.

Does Matter + Energy = LIFE?

At first glance the topic of life seems far removed from our discussion of the basic physics of the cosmos. However, all living things behave according to the laws of physics, no matter what or where they are. In 1917 the Scottish biologist D'Arcy Thompson noted in the introduction to his classic book, On Growth and Form: "Cell and tissue, shell and bone, leaf and flower, are so many portions of matter, and it is in obedience to the laws of physics that their particles have been moved, moulded and conformed." Thompson also recognized that those laws extend beyond the bounds of Earth: "Everywhere Nature works true to scale, and everything has its proper size accordingly. Men and trees, birds and fishes, stars and star-systems, have their appropriate dimensions . . . ." Thompson wasn't suggesting that life might exist elsewhere. Nevertheless, his thoughts provide a framework for our scientific inquiries into extraterrestrial life.

Thompson's observations cast a biological light upon two principles that guide our understanding of the physical nature of the universe. The Copernican principle, named for Nicolaus Copernicus, asserts that Earth is not the center of the cosmos. Rather, it is an ordinary place. Planets like Earth probably circle stars like our Sun throughout our galaxy and in similar galaxies elsewhere. We don't yet have the means to detect Earth-like planets, but we may within the next decade or two. On a more sweeping scale, the cosmological principle holds that the universe is the same (continued)