Cosmos: Chapter 2 - One Voice in the Cosmic Fugue
In this chapter, Sagan ponders the questions of how life came into existence on this planet, how we and the other forms of life on Earth evolved into our present forms, what possibilities exist for life to have come forth on other planets, and how those other forms of life may be both similar and different from Earth-based life forms. Sagan says, "The nature of life on Earth and the search for life elsewhere are two sides of the same question -- the search for who we are."
Sagan points out that all life on Earth is closely related because we are all constructed from instructions encoded in a common language, namely DNA. He asks us to think of Earth-based life as a single "voice" in a "cosmic fugue" of various life forms which arose and evolved independently throughout the cosmos. These other life forms would be very different from us because they would not be based on DNA molecules, which is the only basis for life that we have yet encountered.
"We [all life on Earth] have a common organic chemistry and a common evolutionary heritage. As a result, our biologists are profoundly limited. They study only a single kind of biology, one lonely theme in the music of life. Is this faint and reedy tune the only voice for thousands of light-years? Or is there a kind of cosmic fugue, with themes and counterpoints, dissonances and harmonies, a billion different voices playing the life music of the Galaxy?"
Sagan goes on to describe how evolution, based on either artificial or natural selection, works to mold new life forms over time. He uses a fascinating example to explain artificial selection. The Heike crab, which today thrives in the Inland Sea of Japan, has a shell which bares a remarkable resemblance to the face of an ancient Samurai warrior, complete with eyes, nose, cheekbones and mouth. "How does it come about," Sagan asks, "that the face of a warrior is incised on the carapace of a crab?" Sagan explains that in 1185 a samurai tribe known as the Heike lost a key battle to a rival tribe on the Inland Sea. According to legend, the dead Heike warriors and their emperor wander the bottom of the sea in the form of crabs. Local fishermen, believing this legend, do not eat any crab whose shell resembles a samurai warrior. Thus, crabs with samurai-like shells are artificially selected and "eventually there come to be a great many samurai crabs."
Sagan explains that the same process of artificial selection has given us dairy cows with distended utters, sheep dogs with keen herding instincts, corn stalks with large ears and many other forms of plants and animals bred soley for the benefit of human beings.
Sagan then explains that natural selection (i.e., selection of traits based on survival rather than human preference) is the basis for the evolution of life from its humble beginnings, as first discovered by Charles Darwin and Alfred Russel Wallace. He traces the evolution of the major forms of life on Earth over the last four billion years from the earliest pre-celled forms through present day humans. In a little over three pages he takes us from the chemical composition of the earth before life began, into the earliest pre-celled life forms, and then through one-celled plants, the generation of an oxygen-rich atmosphere, the Cambrian explosion of sea-based life forms, the appearance of fish and vertebrates, land plants, insects, amphibians, reptiles, dinosaurs, mammals, birds, primates and humans.
Sagan then talks about how our evolutionary history has made us cousins to all other forms of life on Earth and how our DNA is based on the same chemical building blocks as the DNA of "trees and angler fish and slime molds and paramecia." He gives a wonderfully clear description of how DNA is constructed, how it replicates itself and how errors in the replication give rise to mutations and thus are the basis for evolution to take place.
Sagan explains that the chemical building blocks of DNA are organic molecules known as amino acids. He then asks how molecules as complex as amino acids could have developed from the ingredients present in Earth's early atmosphere. Easily, he says. He describes an experiment that he and others have conducted in which they mixed the appropriate elements -- hydrogen, water, ammonia, methane, etc. -- into a flask and subjected the mixture to either electrical sparks (simulating lightning) or ultraviolet radiation (simulating sunlight unfiltered by ozone.) After only a few hours, the inside of the flask becomes streaked with what Sagan calls "great gobs" of complex organic molecules.
Sagan concludes the chapter by pondering the possible forms that life on other worlds may take. He says that common notions of extraterrestrial life forms are probably too close to Earth-based life. In his words, "I do not think life anywhere else would look very much like a reptile, or an insect or a human -- even with such minor cosmetic adjustments as green skin, pointy ears and antennae." As an alternative, he describes potential life forms on an imaginary Jupiter-like planet which consists of a very deep atmosphere and little or no solid core. He describes "sinkers", which are very small creatures which sink slowly down through the atmosphere, reproduce quickly and hope that their offspring are carried upwards on convection currents before being carried down by gravity to a hot, high-pressure core. "Floaters" are huge balloon-like creatures, the size of a city, which float through the atmosphere in herds of thousands. "Hunters" are fast, mobile creatures which prey on the floaters.
Finally, Sagan prophetically mentions an event which just this year, in the form of the "Mars rock" found in Antarctica, apparently happened in Sagan's lifetime. He says, "We can know ourselves better by understanding other cases [of life.] The study of a single instance of extraterrestrial life, no matter how humble, will deprovincialize biology. For the first time, the biologists will know what other forms of life are possible."
Tom |
