The fossil record contains the appearances of millions of life forms. There have been some remarkable changes in life on Earth. As described by T.A Conrad: Race after race resigned their fleeting breath-The rocks alone their curious annals save.”
At 3.85 billion years ago, archaea were alive, reproducing and evolving. They obtained energy for their lives by breaking the chemical bonds within molecules such as CO2, H2O, and N2. Archaea are one of the three major branches of life, along with bacteria and eukarya (which includes plants and animals). Archaea today are found down to 3.5 km(2.3mi) below the surface under pressures over 200 atmospheres and at hot springs and deep-ocean spreading centers with temperatures up to 113C (232F), but they are killed by oxygen. These organisms may be relatives of the earliest life forms on Earth.
Over 3.5 billion years ago, photosynthetic bacteria were removing some of the abundant carbon dioxide (C02) from Earth’s atmosphere and combining it with water using the Sun’s energy. The process of photosynthesis gives off oxygen (O2) and thus began the radical transformation of the Earth’s atmosphere that ultimately led to its present composition.
Organisms of these early times reproduced by simple division of their cells. Thus, if a gene had an advantageous mutation, it was limited to a single line of offspring. By I billion years ago, sexual reproduction had appeared. With the arrival of sex, cells could share and mix genetic material and thus speed up evolutionary changes by thousands of times.
During the Earth’s early history, organisms lived primarily below the surface in the oceans and within sediments and rocks. As oxygen (O2) given off during photosynthesis built up to a large volume in the atmosphere, some was altered to ozone (O3), thus building a shield from the Sun’s lethal ultraviolet radiation. With a protective atmospheric shield, more multicellular life could come out into the open. Marine rocks from 670 million years ago contain the oldest known fossils of multicellular animal life, including nearly all the major body plans that exist today.
About 544 million years ago, life on Earth began a 40 million-year-long burst of remarkable evolutionary change. From this time onward, the fossil record improves because many of the new groups of organisms began creating hard parts, such as shells, that preserve well as fossils. The initial development of hard parts has been referred to as “the world’s first arms race” wherein organisms covered their bodies with armor, possibly as protection against predators.
Moving on through time (up the sedimentary rock sequence), the Earth has continued to fill with new forms of life. For example, the waters became home to fishes, while plants moved out of the water and spread across the lands. Later on, vertebrate animals appeared on land-amphibians, then reptiles, and finally the mammals and birds. The evolution and increasing diversity of species is continuing.
Today, life is almost everywhere-in waters ranging from OF to above boiling, on land, in the air, in the soil, in ice, around deep-sea volcanic vents, within the pores of rocks buried several kilometers deep, and inside other organisms. The overall trend has been an increase in the diversity and abundance of life, but there have been major setbacks and reorganizations. Plots the number of families of marine animals with hard skeletons that have lived during the last 600 million years. Over time, there has been an obvious growth in the number of families; this growth reflects the increase in diversity and abundance of marine life. However, there also have been marked die-offs in which more than 50 percent of the families becomes extinct within relatively short time intervals of several million years or less. To better understand these dramatic extinction events, it is necessary to look at events on the species level, which is where the real action takes place.
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