rs old while life as we know it emerged about 3.7 billion years ago. During that period, carbon atoms were gradually transformed into complex carbon-based, or organic, compounds, and eventually, organelles, the basic components of living cells, such as mitochondria. The roughly 3.2 billion years that followed-known to paleontologists as the Archaeon and Proterozoic Eons-saw the emergence first of anaerobic and then oxygen-breathing life forms. But these early life forms were simple and largely microscopic, leaving virtually no fossil record behind. p> It was not until the beginning of the Paleozoic era, around 540 million years ago, that more complex plant and animal forms began to appear and leave a fossil record. The earliest period of the Paleozoic era is known as the Cambrian; thus, most paleontologists refer to the time span before complex life forms began to emerge as pre-Cambrian time. For the most part, paleontologists are forced by a lack of a physical record to study life forms from the Cambrian period forward. br/>
History of the Discipline
Among the key issues paleontologists grapple with is how life has evolved on Earth. In that sense, they are examining two key questions that have exercised the human imagination for millennia: why is there such a diversity of life and where did it all come from? Virtually all cultures have myths and stories to answer these questions. To Western readers, the most familiar is that in the book of Genesis-six days in which God first created the physical universe, the Earth, and then populated the latter with animals, plants, and finally human beings. The Book of Genesis also spoke of a planet-wide flood ten generations after Adam and Eve-generations that lasted hundreds of years each - but it noted that all of Earth's creatures were saved by Noah in his ark: "every animal, every creeping thing, and every bird, everything that moves on the earth, went out of the ark " after the flood.
This biblical explanation, of course, left no room for fossils. Among the earliest thinkers to wonder about this natural phenomenon was the Greek philosopher Xenophanes in the sixth century BCE. Examining the fossils of shellfish, Xenophanes assumed they were the remains of existing species though he was required to come up with an explanation for why they were found so far from the sea. Xenophanes hypothesized that land forms shift. The eleventh century CE Chinese scientist Shen Kuo explained the presence of bamboo fossils in dry climates incapable of supporting that particular species by a theory of climate change over time.
But not all scholars concurred with these findings. As late as the sixteenth century, most European thinkers questioned whether fossils were even evidence of life at all, assuming that fossils, though lifelike in appearance, were simply odd-looking stones. Indeed, the original Latin meaning of the word fossil was simply "something dug from the Earth," with no implicit meaning that the things being dug up had once been life forms. Ancient and medieval Chinese came to a different conclusion about the dinosaur bones that they found, explaining them away as evidence of the mythical creatures, dragons, which, they believed, still existed in faraway places.
With the rise of the so-called Age of Reason and the Scientific Revolution of the seventeenth century, many European thinkers began to seek non-theological explanations for natural phenomena. In 1665, the English scientist Robert Hooke, utilizing the newly invented microscope, put forth the theory of a mineralization process to explain petrified wood. Such a process assumed a much greater time span for life than that offered in the Bible. Roughly a century later, French naturalist Georges Buffon explained the existence of fossilized elephant bones in Europe by saying that the Earth was undergoing a gradual cooling process over time, since tropical elephants no longer lived in a temperate Europe.
The greatest breakthrough of the pre-Darwinian era in paleontology, however, came with the findings of Cuvier. Utilizing the newly invented species classification system of Swedish scientist Carl Linneaus, Cuvier established that existing elephant species differed from the elephant-like creature, which he named the mastodon, whose fossilized bones had been discovered in North America's Ohio Valley region. Cuvier then hypothesized that this species was extinct, thereby undermining both the idea that fossils were the bones of existing species and Buffon's cooling Earth theory. Instead, Cuvier put forth the theory of catastrophism, that sudden geological changes explained extinction. This undermined another existing paradigm, known as uniformitarianism, which stated that geological change occurred gradually and uniformly through time. These various findings have led historians of science to consider the French naturalist the father of...
