when did evolution start
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All life on Earth evolved from a single-celled organism that lived roughly 3.5 billion years ago, a new study seems to confirm.
The study supports the widely held “universal common ancestor” theory first proposed by Charles Darwin more than 150 years ago.
(Pictures: “Seven Major ‘Missing Links’ Since Darwin.”)
Using computer models and statistical methods, biochemist Douglas Theobald calculated the odds that all species from the three main groups, or “domains,” of life evolved from a common ancestor—versus, say, descending from several different life-forms or arising in their present form, Adam and Eve style.
The domains are bacteria, bacteria-like microbes called Archaea, and eukaryotes, the group that includes plants and other multicellular species, such as humans.
The “best competing multiple ancestry hypothesis” has one species giving rise to bacteria and one giving rise to Archaea and eukaryotes, said Theobald, a biochemist at Brandeis University in Waltham, Massachusetts.
But, based on the new analysis, the odds of that are “just astronomically enormous,” he said. “The number‘s so big, it’s kind of silly to say it”—1 in 10 to the 2,680th power, or 1 followed by 2,680 zeros.
History of evolutionary theory
All human cultures have developed their own explanations for the origin of the world and of human beings and other creatures. Traditional Judaism and Christianity explain the origin of living beings and their adaptations to their environments—wings, gills, hands, flowers—as the handiwork of an omniscient God. The philosophers of ancient Greece had their own creation myths. Anaximander proposed that animals could be transformed from one kind into another, and Empedocles speculated that they were made up of various combinations of preexisting parts.
Closer to modern evolutionary ideas were the proposals of early Church Fathers such as Gregory of Nazianzus and Augustine, both of whom maintained that not all species of plants and animals were created by God; rather, some had developed in historical times from God’s creations. Their motivation was not biological but religious—it would have been impossible to hold representatives of all species in a single vessel such as Noah’s Ark; hence, some species must have come into existence only after the Flood.
Charles Darwin
The founder of the modern theory of evolution was Charles Darwin. The son and grandson of physicians, he enrolled as a medical student at the University of Edinburgh. After two years, however, he left to study at the University of Cambridge and prepare to become a clergyman. He was not an exceptional student, but he was deeply interested in natural history. On December 27, 1831, a few months after his graduation from Cambridge, he sailed as a naturalist aboard the HMS Beagle on a round-the-world trip that lasted until October 1836. Darwin was often able to disembark for extended trips ashore to collect natural specimens.
The discovery of fossil bones from large extinct mammals in Argentina and the observation of numerous species of finches in the Galapagos Islands were among the events credited with stimulating Darwin’s interest in how species originate. In 1859 he published On the Origin of Species by Means of Natural Selection, a treatise establishing the theory of evolution and, most important, the role of natural selection in determining its course. He published many other books as well, notably The Descent of Man and Selection in Relation to Sex (1871), which extends the theory of natural selection to human evolution.
Modern conceptions
The Darwinian aftermath
The publication of the Origin of Species produced considerable public excitement. Scientists, politicians, clergymen, and notables of all kinds read and discussed the book, defending or deriding Darwin’s ideas. The most visible actor in the controversies immediately following publication was the English biologist T.H. Huxley, known as “Darwin’s bulldog,” who defended the theory of evolution with articulate and sometimes mordant words on public occasions as well as in numerous writings. Evolution by natural selection was indeed a favourite topic in society salons during the 1860s and beyond. But serious scientific controversies also arose, first in Britain and then on the Continent and in the United States.
The synthetic theory
The rediscovery in 1900 of Mendel’s theory of heredity, by the Dutch botanist and geneticist Hugo de Vries and others, led to an emphasis on the role of heredity in evolution. De Vries proposed a new theory of evolution known as mutationism, which essentially did away with natural selection as a major evolutionary process. According to de Vries (who was joined by other geneticists such as William Bateson in England), two kinds of variation take place in organisms. One is the “ordinary” variability observed among individuals of a species, which is of no lasting consequence in evolution because, according to de Vries, it could not “lead to a transgression of the species border [i.e., to establishment of new species] even under conditions of the most stringent and continued selection.”
An Oxygen Atmosphere
Evidence for this Great Oxidation Event is recorded in changes in seafloor rocks. When oxygen is around, iron reacts chemically with it (it gets oxidized) and gets removed from the system. Rocks dating to before the event are striped with bands of iron. Rocks dating to after the event do not have iron bands, showing that oxygen was now in the picture.
Multicellular Life
However, other innovations were occurring. While they can process lots of chemicals, microbes did not have the specialized cells that are needed for complex bodies. Animal bodies have various cells –skin, blood, bone – which contain organelles, each doing a distinct job. Microbes are just single cells with no organelles and no nuclei to package their DNA.
Something revolutionary happened as microbes began living inside other microbes, functioning as organelles for them. Mitochondria, the organelles that process food into energy, evolved from these mutually beneficial relationships. Also, for the first time, DNA became packaged in nuclei.
The First Animals
These clusters of specialized, cooperating cells eventually became the first animals, which DNA evidence suggests evolved around 800 million years ago. Sponges were among the earliest animals. While chemical compounds from sponges are preserved in rocks as old as 700 million years, molecular evidence points to sponges developing even earlier.
Oxygen levels in the ocean were still low compared to today, but sponges are able to tolerate conditions of low oxygen. Although, like other animals, they require oxygen to metabolize, they don’t need much because they are not very active. They feed while sitting still by extracting food particles from water that is pumped through their bodies by specialized cells.
Ediacaran Biota
By about 580 million years ago (the Ediacaran Period) there was a proliferation of other organisms, in addition to sponges. These varied seafloor creatures – with bodies shaped like fronds, ribbons, and even quilts – lived alongside sponges for 80 million years. Their fossil evidence can be found in sedimentary rocks around the world.
However, the body plans of most Ediacaran animals did not look like modern groups. Smithsonian’s Dr. Douglas Erwin, using comparative developmental evidence, has examined whether any of the fossilized Ediacaran animals were related to modern animals.
The End-Ediacaran Extinction
However, about 541 million years ago, most of the Ediacaran creatures disappeared, signaling a major environmental change that Douglas Erwin and other scientists are still working to understand. Evolving animal body plans, feeding relationships, and environmental engineering may have played a role.
Burrows found in the fossil record, dating to the end of the Ediacaran,
reveal that worm-like animals had begun to excavate the ocean bottom.
The Cambrian Explosion
The Cambrian Period (541-485 million years ago) witnessed a wild explosion of new life forms. Along with new burrowing lifestyles came hard body parts like shells and spines. Hard body parts allowed animals to more drastically engineer their environments, such as digging burrows. A shift also occurred towards more active animals, with defined heads and tails for directional movement to chase prey.
Active feeding by well-armored animals like trilobites may have further disrupted the sea floor
that the soft Ediacaran creatures had lived on.
Spontaneous generation
Life ultimately is a material process that arose from a nonliving material system spontaneously—and at least once in the remote past. How life originated is discussed below. Yet no evidence for spontaneous generation now can be cited. Many believed in spontaneous generation because it explained such occurrences as maggots swarming on decaying meat.
Geologic record
Past time on Earth, as inferred from the rock record, is divided into four immense periods of time called eons. These are the Hadean (4.6 billion to 4 billion years ago), the Archean (4 billion to 2.5 billion years ago), the Proterozoic (2.5 billion to 541 million years ago), and the Phanerozoic (541 million years ago to the present). For the Hadean Eon, the only record comes from meteorites and lunar rocks. No rocks of Hadean age survive on Earth. In the , eons are divided into eras, periods, and epochs. Such entries in the geologic time scale are often called “geologic time intervals.”
