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Fire and Ice
Microscopic life evolved some 3.5 billion years ago in the Precambrian period during the first and longest chapter of life that covers about 90 percent of geological time. No one knows exactly what the earth was like when microbial life began but we do know that at some time the earth was a hot and hellish place with an atmosphere that lacked oxygen. Early microbes, probably bluegreen algae or bacterialike organisms, invented photosynthesis to harness sunlight as a source of energy. They took carbon dioxide out of the air as their food, and they generated oxygen as a waste product that further transformed the atmosphere and hence the climate. They developed DNA for storing information, invented sex, which produced variation for natural selection, and evolution took off on its unending and largely unpredictable course.
Molecular fingerprinting suggests that every life-form on earth today originated from the same bacterialike ancestor. That ancestor eventually led to the three main surviving branches of life, the archaea, bacteria, and the eukaryotes (the organisms made of cells with a nucleus that include algae, plants, fungi, and animals).
Remnants of the first ancient pre-oxygen-using life may still exist little-changed today. They are thought to be sulphur-consuming bacteria now living only in the few remaining places where the ancient and to us hellish conditions still remain. These habitats include hot springs and deep oceanic thermal vents where water at 300°C (that stays liquid there rather than turning to steam because it is under intense pressure in depths of some 3,600 meters) issues up from the ocean floor. One of the species living at the edge of these hot water vents is Pyrolobus fumarii, which can't grow unless heated to at least 90°C, and which it tolerates 113°C. As the earth cooled new environments became available and new single-celled and then multicelled organisms evolved from these or similar species to invade ever-new and cooler environments.
Some cells much later also escaped their ancestral conditions by invading other cells, finding that environment conducive for survival and adapting to it. Such initially parasitic organisms ultimately evolved into cooperative or symbiotic relationships with their hosts. Perhaps the most fateful of these eventually mutually beneficial associations occurred when some Precambrian green algae successfully grew inside other cells, to ultimately become chloroplasts, while their hosts then became green plants.
The ability to capture solar energy that ushered in the multicellular life and the fantastic diversity of life we see today was followed by or concurrent with one other critical parasitic-turned-symbiotic cellular invasion. The availability of oxygen from plants led to energy and oxygen-guzzling bacteria, and when some of these invaded other cells they became mitochondria and their hosts became animals.
Mitochondria are the cell's source of power or energy-use, and having mitochondria with access to oxygen allowed vastly greater rates of energy expenditure. It made the evolution of multicellular animals possible. One of the ultimate expressions of the high-energy way of life that is powered by the use of mitochondria is, of course, animals like the kinglets that maintain a liveness at an, to us, almost unimaginably high and sustained rate through a northern winter.
The metabolic fires generated by the mitochondria can be fanned to run on high, given the availability of much oxygen, or they may be turned down low. Life is the process that harnesses, and more importantly, controls that fire. It produces heat, and heat is often synonymous with life.
Temperature is, to us, a sensation measured on a scale of hot to cold. Physically, it is molecular motion, and we can measure it with a thermometer because the greater the motion of the molecules of a substance, say mercury, the farther apart they are spaced. We measure this molecular expansion as mercury (or some other liquid) in a column is displaced up a calibrated scale. The molecular motion, as such, is not life but a prerequisite for it.
Heat, on the other hand, is the energy that goes in or out of the system to change temperature. Some substances must absorb more energy (from the sun for example) before their molecules are set into motion, raising the temperature. One calorie is the unit of energy defined to raise one gram of water one degree Celsius. Substances, like rock, heat up with much less energy than that required to heat water. Again, energy is not life, but a prerequisite for it, and life is insatiable for it. What is truly miraculous, therefore, is that life continues and even thrives in winter, when the sun is low.
There is no upper limit of temperature. Within our solar system, the surface temperature of the sun is about 6,000°C; the center is about 3,000 times higher, or 18,000,000°C. The lower temperature limit in the universe, on the other hand, is finite. It's the point at which all molecular motion stops and the heat energy content is zero. That temperature precludes living, but from adaptations to the winter world that I will discuss, it need not destroy life. Life can, at least theoretically, persist on hold at the lowest temperature in the universe.
Our centigrade scale is defined as a 100-arbitrary-unit division of heat energy content of water, between when water molecules leave the crystal structure to become liquid (0°C) and 100°C when the liquid water boils at sea level. The zero energy content of matter, or lowest temperature limit in the universe, is defined as 0°K on the Kelvin scale and it corresponds to -273.15°C or -459.7° on the Fahrenheit scale. Since life as we know it is water-based, the active cellular life that most of us are familiar with is restricted to the very narrow temperature range between the freezing and boiling points of water (which vary somewhat depending on pressure and presence of dissolved solutes) where the controlled rates of energy use become possible. We are composed mostly of water ...