Astronomers have plotted the trajectories of various

This preview shows page 7 - 8 out of 10 pages.

Astronomers have plotted the trajectories of various stars and galaxies and determined that all matter in the known universe arose from a common point. In an event, often referred to as the "Big Bang," the universe arose in a relatively brief moment in time and matter was flung outward from the central origins. Matter has been hurling through space and undergoing a number of changes as it traverses the cosmos. Our solar system formed about 4.5 billion years ago. It appears that Earth began forming shortly thereafter, and evidence suggests that life first appeared by the end of the first billion years. The available evidence indicates that evolutionary events have been ongoing over the last 3.5 billion years and that these processes have led to the diversity of life we see around us today. In thinking about evolution, remember that all extant (living) species represent "modern" forms of life. The challenge that faces evolutionary biologists is to reconstruct events that have occurred in Earth's distant past, and to infer the nature of ancient events that have contributed to the current diversity of organisms. Most researchers who tackle questions on the origin of life think that organic compounds first formed abiotically. The experiments of Urey and Miller demonstrated that these compounds could have formed in an atmosphere similar to that which existed on early Earth. It is also possible that organic compounds might have formed in deep-sea vents. Another hypothesis, panspermia, is that organic compounds (or perhaps life itself) arrived on the planet from extraterrestrial sources. One of the first challenges is to know the temporal sequences of historical events. Fortunately, geologic events provide valuable information. The Earth's crust is not a static structure. It changes as a result of plate tectonics (which leads to continental drift, which is measured at about 1 inch/year, on average), as well as changes in the water levels of the oceans (due to redistributions during one of several ice ages), wind, the successive layering of sediments (material in the oceans sink), and erosion (wind and water pound on the exposed terrestrial ground). The bottoms of these ancient oceans exist in the form of sedimentary rock in many places on the planet. Many extinct species have left their fossilized impressions in these sedimentary layers. Geologists have studied these layers around the world and have noted common species' patterns of fossils in these various layers. Generally speaking, the deeper the layers, the older the sediment and the more ancient the fossilized species. This relationship is so pervasive that geologists use fossils to describe the layer they are examining. (This has certain practical uses, as geologists looking for oil use the presence of certain fossilized species to know when their drills are approaching oil-bearing strata.) The geological time scale is based on the relative distribution of fossils in sedimentary layers. A close examination of these layers reveals various patterns in the distribution of various fossilized species at different depths (i.e., relative ages). Various

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture