Earth is estimated to be around 4.6 billion years old, but the earliest evidence of life dates back to around 3.8 billion years ago. Until about four billion years ago, Earth's environment was too intensely hot and rapidly changing to support life. Since life formed, major trends and patterns of evolutionary change have emerged. Photosynthetic cyanobacteria, an early form of bacteria, oxygenated the planet, paving the way for oxygen-using eukaryotes. Fish have inhabited the oceans for millions of years. One such fish is the coelacanth, which first appeared in the geologic record about 360 million years ago and which was believed to have become extinct about 65 million years ago. An individual coelacanth from one species, Latimeria chalumnae, was discovered in 1938. A coelacanth from another species, L. menadoensi, was found living in waters near Indonesia in 1998. During the time span that coelacanths have survived, there have been two mass extinctions as well as the rise of limb bones, weight-bearing joints, four-legged animals, and hominids. A mass extinction occurs when many species are lost because of global environmental conditions. Modern tetrapods share a recent common ancestor with finned, gill-bearing fish, including the coelacanth, lungfish, and ray-finned fish.
Evidence for macroevolution comes from a variety of scientific fields, including geology, paleontology, and molecular genetics. The events that mark macroevolution are determined by using scientific evidence. Geologic evidence includes the formation of rock layers, the fossil record, and comparisons to living organisms. The fossil record includes all the fossilized artifacts taken in the context of their placement within Earth's geological strata. Molecular evidence includes mapping and comparing the genomes of different organisms to look for similarities in their genetic codes. Genetic evidence can also be used to provide information about extinct animals, such as the wooly rhinoceros. This animal is believed to have lived about 39,000 years ago and was well-adapted for living in cold conditions.Identifying specific evolutionary events is just the beginning of understanding macroevolution. Scientists study specific mechanisms of microevolution (a change in allele frequencies within a population from one generation to the next) to enhance their understanding of macroevolution. Mechanisms of microevolution and speciation, the process through which a species evolves into two or more new species, result in macroevolution on a geologic timescale.
Life Begins on Earth
About four billion years ago, Earth's lithosphere cooled sufficiently for solid rocks and liquid water to form. Oceans were unlike the oceans that exist today, being more acidic due to the acids from the atmosphere dissolving in the water. The early atmosphere was unlike today's atmosphere, being comprised mostly of carbon dioxide, sulfur dioxide, and hydrogen sulfide. These gases were spewed forth from erupting volcanoes. Low oxygen levels encouraged the formation of small organic molecules because oxygen tends to break chemical bonds. The primordial soup theory suggests that life began in one of the ancient oceans or another body of water because of the chemical combination in the atmosphere and the influx of energy to make molecules that support life, such as amino acids, which are the building blocks of proteins. The primordial soup theory gained acceptance in the 1950s, when biochemists Stanley Miller and Harold Urey conducted an experiment simulating early Earth's atmospheric conditions. This experiment demonstrated that several organic compounds could be formed spontaneously under these conditions. Today, many scientists suggest the waters of early Earth were anoxic—depleted of oxygen—and that cyanobacteria produced oxygen gas. When these organic molecules first formed, the atmosphere consisted mainly of CO2 and nitrogen, with almost no oxygen available. A modern study suggests that biological molecules may have originated in deep sea vents because deep sea vents have chemicals that could have provided the energy for building molecules.
A protocell is a self-organizing, spherical collection of lipids proposed as the origin of living cells. The first protocells formed in the ocean. Scientists theorize that protocells contained a membrane composed of fatty acids surrounding a replicating center of ribonucleic acid (RNA). The RNA world hypothesis suggests that self-replicating RNA was a precursor to DNA and proteins. This idea was supported by studies that found that clay can assist in the formation of RNA strands and membranes. According to this theory, protocells containing the RNA-replicating centers eventually evolved to use DNA and proteins. RNA is relatively unstable and has poor catalytic properties, so ribozymes (RNA-based catalysts) were gradually phased out in favor of DNA.
The basic needs of life then were not dissimilar from the needs life requires now. These needs included a source of energy, a way to capture and process that energy, and a mechanism to control bodily processes. In addition, the presence of protocells was essential. Over time mutations of these protocells occurred, resulting in cells that replicated quickly. The ability to replicate was the key to the evolution of prokaryotes at about 3.8 billion years ago. The ability to replicate allowed for the passage of favorable traits that made offspring more suited to their environment.
Microscopic life forms evolved from replicating protocells, and some of these early life forms left fossil evidence. Stromatolites, rock-like structures made by cyanobacteria and other microbes, are fossil evidence of these microscopic life forms. Clusters or mats of early microbes collected particles of sediments. These microbes became bound up with sediment and formed clumps, mounds, columns, or strata of sedimentary rock.
History of Life
The endosymbiotic theory is the idea that eukaryotic cells and their organelles evolved from early cells engulfing prokaryotes. Millennia ago, a host cell and bacteria that it ingested could have become dependent upon one another for survival. Over millions of years, this relationship evolved into a permanent one. Through this evolution, mitochondria, the cell's energy organelle, and chloroplasts, which are even more specialized and which today cannot live outside the cell, arose. Eukaryotic cells evolved through symbiotic relationships between bacterial cells and proto-eukaryotic cells. This allowed single-celled organisms to develop greater complexity and form multicellular organisms, all of which lived in the oceans. Three lines of multicellular organism evolved in water: plants, animals, and fungi. By 1,300 million years ago, fungi had become established on land and land plants evolved by about 700 million years ago. The first invertebrate animals evolved around 700 million years ago, and the first vertebrate animals appeared around 550 million years ago.By the Cambrian period, 541–515 million years ago there was a remarkable event of evolutionary diversification that filled the seas. The Cambrian explosion (541 million years ago) was a biologic event during the Cambrian period when many lineages of organisms first appeared. It brought about the evolution of eukaryotic lineages, such as marine worms that breathed through gills, arthropods (invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages)—including crustaceans such as crabs, shrimps, lobsters, krill, barnacles, and crayfish—trilobites (marine arthropods), and several predators, such as the giant, shrimplike Anomalocaris, which trapped its prey using mouthparts lined with hooks. and the five-eyed Opabinia, which caught its victims using a flexible clawed arm attached to its head. Scientists estimate that species biodiversity during the Cambrian explosion may have reached the level of biodiversity seen today, though the species were different and their environments were different. Most of those species are extinct today.