Discovering Fossils

Stages of Life on Earth

Life has been evolving on Earth for close to four billion years. The earliest life on Earth appeared during the Precambrian super eon, which began when the Earth was formed and ended about 550 million years ago. Fossils show that simple cells, known as prokaryotes, appeared on Earth between about three and a half and four billion years ago. The stage of the Earth's history after the Precambrian, in which we are living today, is known as the Phanerozoic eon. Phanerozoic means "visible life". Multicellular plants and animals became abundantduring the Phanerozoic eon. The Phanerozoic eon is divided into three eras: the Paleozoic, the Mesozoic and the Cenozoic eras. Paleozoic means "ancient life". The Paleozoic era lasted from about 550 million to about 250 million years ago. Life forms that arose during the Paleozoic include animals such as insects, crustaceans, fish, amphibians and reptiles, and plants such as ferns, conifers and cycads. Many of the animals that flourished during the Paleozoic are now extinct. The Mesozoic ("middle life") era lasted from about 250 million to about 65 million years ago. Dinosaurs lived during the Mesozoic era. Mammals, birds and flowering plants appeared on Earth for the first time. The latest stage of life on Earth has been the Cenozoic era. Cenozoic means "modern life". The Cenozoic era, which began about 65 million years ago, is the geologic era that we are living in now. During the Cenozoic era, insects, mammals, birds, and flowering plants became widespread. Humans appeared on Earth in the last few million years of the Cenozoic.

X-Woman

In March 2010, a pinky bone from a young hominin – a member of the group of animals that includes humans, chimpanzees and bonobos (sometimes known as pygmy chimpanzees) – was discovered in Denisova Cave in Altay Krai in southern Siberia. Carbon dating of artifacts found in the cave, including a bracelet, shows that the owner of the pinky bone, who became known as X-woman, probably lived between about 30,000 and 50,000 years ago. X-woman was probably between 5 and 7 years old when it died. DNA from the X-woman's mitochondria (structures in cells that are involved with energy, respiration and growth) was compared to the mitochondrial DNA (mtDNA) of 54 modern humans from Russia, the mtDNA of six Neanderthals, and the mtDNA of one chimpanzee and one bonobo. (The name X-woman comes from the fact that mitochondrial DNA passes only through the female line; we don't know the actual sex of X-woman.) X-woman's mtDNA was found to be twice as different from that of modern humans as the mtDNA of Neanderthals is from us. Modern humans, Neanderthals and the speciesto which X-woman belonged had a common ancestor that lived about one million years ago. Scientists think that modern humans migrated from Africa about one million years ago – after Homo erectus, an ancestor of modern humans, left Africa and before the ancestors of Neanderthals migrated from Africa. X-woman's species, Neanderthals and modern humans may have interacted with one another and may even have interbred. In fact, X-woman may have been a hybrid – with a father who was a modern human or a Neanderthal. X-woman's mitochondrial DNA tells us nothing about its male ancestry. Researchers believe that X-woman's species probably hunted wooly rhinos and wooly mammoths and wore heavy clothing to keep warm. The discovery of X-woman shows that at least four different species of human beings – Homo sapiens (modern humans), Homo neanderthalensis (Neanderthals), Homo floresiensis, and X-woman – have lived in Europe and Asia. This is the first time that DNA sequencing – rather than the structure of fossils – has been used to describe a hominid. (Hominids include humans, chimpanzees, bonobos, orangutans and gorillas.)

Discovering Fossils

In 1887 Eugene Dubois, a young Dutch physician, uncoveredon the island of Java in what was then the Dutch East Indies the fossil bones of a specimen he called Pithecanthropus erectus (the upright ape-man). Java man, as he has since become known, possessed a flattish skull somewhat apelike in appearance. The shape of his thigh bone, though, indicated that he had walked upright, like a human. Public outrage ran high at Dubois' claim to have found an early ancestor of man. Many people could not accept the possibility that modern man had descended from such primitive stock. Shortly after the discovery of Java man, however, a similar fossil skull was found in a cave at Zhoukoudian, near Beijing. It was called the Peking (former name of Beijing) man. The two pieces of evidence at such widely separated points in Asia suggestedthat a species of man more primitive than Homo sapiens had once ranged over much of the Orient.

In 1924 Dr. Raymond Dart, a professor of anatomy at South Africa's University of Witwatersrand, identified a fossil from a limestone quarry at Taung as the skull of a six-year-old child with a brain case no larger than that of a young ape, but with other clearly human traits. Not until 20 years later were enough adult skulls of this same type recovered by another South African paleontologist, Dr. Robert Broom, to confirm that these early creatures were not slightly unusual chimpanzees but higher forms of primates more closely alliedto humans, despite their small brain case and small stature. These creatures are known as Australopithecines (southern apes). The potassium-argon method of radio­active dating has found some of the Australopithecine remainsto be as old as 2.5 million years. It is in this same region of Africa where the Australopithecines once flourished that more advanced and recognizably manlike beings eventually are believed by anthropologists to have arisen, probably as descendants of the Australopithecines. If the Australopithecines can not quite qualify as the first men on the Earth, their descendants do and it is widely accepted that all humans originally descended from ancestors in what is now Africa.