From the Earth's Origin through Human Existence

 The earth is consisting of solid particles drifting in the space. How can we know the earth's age? The original solid particles, that is, minerals have been changed by the incessant activity of the earth. However, meteorites without such effect are considered as the best specimens to estimate the earth's age. From the radioactive isotope dating on meteorites, the formation of the earth is estimated about 4.6 billion years ago.

 Stratified beds are being formed in every environment on the Earth at any time. The fossils in the beds are the evidence for the history of the Earth's environments and evolution of life. Based on the changes of fossil species through the stratified beds, the evolutionary history of life, such as the origin, diversity, and extinction, can be inferred. The worldwide geologic time table has been made based on the transition of the fossil records.

In the past 600 million years, the number of biotic species has been drastically reduced on at least five occasions. Extinctions took place at the end of the Cretaceous Period, 65 million years ago; they were probably caused by a collision between the Earth and an asteroid-sized body, accompanied by subsequent global cooling. The extinctions that occurred at the end of the Permian Period, 250 million years ago, constituted the greatest life crisis ever. The end-Permian extinctions are likely to have resulted from massive volcanic eruptions that accompanied the fragmentation of the Pangaea super-continent, and a succeeding an oxidization of the oceans.

Sedimentary rocks that formed on the bottoms of oceans and lakes often contain minute fossils. These microfossils consist of the rests of small planktonic and benthic organisms, the teeth and scales of fishes, and pollens and spores. Such microfossils are widespread in rocks and are used to assign ages to sedimentary rocks and to reconstruct the Earth's past environments.

In the Precambrian time, about 3 billion years ago, the biosphere was dominated mainly by bacteria and other micro-organisms. As a result of the photosynthetic activity of these micro-organisms, stromatolites developed, with the liberation of oxygen into the ocean. The liberated oxygen in the ocean combined with iron to from banded-iron formations. Most Precambrian soft bottoms were sealed by microbial mats and today traces of their activities are recognizable on the surfaces of rocks.

Macroscopic organisms first appeared in the Vendian age (650 to 540 million years ago) of the latest Precambrian time. These fossil organisms have been know worldwide in such areas as Ediacara of Australia and look strange in shape. Some paleontologists have pointed out that many of the Vendian organisms possess unique body structures and belong to an independent group of organisms (called  Vendobionta) unrelated to any known organisms today.

Remarkable fossil biotas of Cambrian age (ca. 520 million years ago) have been recovered from the Burgess Shale of British Columbia, Canada, strata of Chengjian area, Yunnan Province of China, among others. Many of these fossils, like Anomalocaris, exhibit body structures quite different from those of modern animals. Such fossil animals tell us an event of evolution in a way of  "try and error" in the sea of the Cambrian Period.

At the beginning of early Paleozoic Cambrian time, some 540 million years ago, animals underwent "explosive evolution", and most of the major groups of life that are known from the present day appeared. They evolved in various ways during the Plaeozoic, and many strange-shaped creatures such as "Tully monster" inhabited in the sea. However, almost all of the animals became extinct by the end of the Paleozoic.

The trilobite, one of the arthropod groups, had extensively diversified in the early Paleozoic sea. Morphological characters of the trilobite are fairly diverse and some of them are so peculiar, for example possessing fork-like head projections. Because of the extensive diversification relative to the other invertebrates and the diverse morphological characters, one may call them as the king of the Paleozoic sea. However, we are far from glass clear in understanding how such characters were related to particular life habits. They are no longer survived after the termination of the Paleozoic era which is about 250 million years ago. 

According to fossil records, the most primitive fishes which first acquired vertebrae appeared in the Early Cambrian Period (about 530 million years ago). At the beginning, fishes did not have jaws, but when they acquired jaws, fishes rapidly diversified. In the Devonian Period, not only all major taxonomic branches of fishes but also amphibians appeared. In the process of the evolution, fishes have developed jaws, skeletons of limbs, lungs which made possible descendants to live terrestrial life.

Synapsids and reptiles evolved from an advanced group of Amphibians as the common ancestor during the Late Carboniferous (ca. 320 million years ago). Pelycosaurs, the early members of synapsids, are thought to be the ancestors of the mammals., because pelycosaurs have an opening behind the eye socket on the skull, and the same feature can be seen on the skull of mammals. Synapsids continued to evolve, and the true mammals appeared at the Late Triassic (ca. 230 million years ago).

Majority of the Mesozoic mammals had been the size of mice or rats, were nocturnal, and ate worms and insects. Flowering plants appeared in the Early Cretaceous (ca.130 million years ago), the insects that prefer flowers diversified, and the primitive marsupials and placentals who developed most suitable teeth for insect eating appeared. Toward the end of Cretaceous (65 million years ago), primitive ungulates and primates who would have ruled the Cenozoic Era appeared.

At the beginning of the Cenozoic Era, tropical and subtropical forests expanded worldwide. The niches in those forests that became empty after the extinction of dinosaurs were occupied by the survived mammals who became larger and diversified rapidly. Primitive ungulates that ate tree leaves and roots developed on the ground, while primitive primates and rodents diversified on the trees. The Earth became cooler in later stages, new kinds of forest dwellers appeared.

Earth's temperature cooled down suddenly and then gradually since the beginning of the Oligocene (ca. 34 million years ago), and at about the same time, the continental area of middle latitude started becoming grassland with dryer climate. Development of such grassland prompted some of perissodactyls, artiodactyls, and rodents to evolve groups who adapted the grassland. In further dryer area, mammals such as camels who can survive in much dryer environment and mammals who make nests under the ground appeared.

In South America that had been isolated from other continents for long period of time, unique mammals evolved. Marsupials and primitive placentals who already existed in South America by the end of Cretaceous (65 million years ago) and rodents and primates who invaded at the end of Eocene (ca.34 million years ago) evolved uniquely within South America. But, most of those mammals became extinct under the influence of mammals who invaded from North America that was connected with South America at about 3 million years ago.

Among the herbivorous mammals, various ways to protect themselves from carnivores, such as running fast, going underground, climbing trees, and covering body with armors, have been developed. Having huge bodies is one of them. Most characteristic feature of the evolution of elephants is enlargement of the body and getting heavier weight, and unique features appeared on the skeleton and teeth to support such heavy body. During the geologic past, diversified fossil elephants distributed all the continents except for Australia and Antarctica.

Because the herbivorous mammals diversified rapidly after the end of Cretaceous, the carnivorous mammals also evolved quickly. The typical carnivores are the extant order Carnivora, but extinct creodonts were rather predominant during the early Cenozoic. The mesonichids who are somehow close to the ancestors of whales were also carnivorous. In South America, some marsupials became carnivorous and displayed convergent evolution with true carnivorans.

Sauropsida and Synapsida evolving from amphibians in the latter Paleozoic Era soon extended their living areas ashore though some returned to aquatic life. Mesozoic Era aquatic reptiles and Cenozoic Era aquatic mammals evolved by way of different paths to having similar outward appearances. Over a period of two hundred million years in underwater environments convergent evolution occurred on a grand scale.

While reptilian and mammalian evolution on land saw diversification in from of offspring from similar shaped forbears, convergent evolution was taking place in water bodies in which similar shaped offspring arose from differently shaped ancestors. This process occurred through adaptation in response to diet and means of mobility. Comparison of Mesozoic Era aquatic reptiles and Cenozoic Era aquatic mammals show a surprising resemblance in body shape.

Look back at the gallery, on the floor immediately in front of you is the partial skull of the gigantic ichthyosaur. In 1997, paleontologists from the National Science Museum joined an international team in uncovering the skull. It took four summers of fieldwork to excavate all the fossils. This great ichthyosaur, with a total length of 21 meters, lived in the sea about 220 million years ago. It is not only the largest ichthyosaur but also the largest marine reptile ever known to science. Science are examining the ecological and environmental factors that enabled ichthyosaurs to achieve such a large size.

In Africa about 6 million years ago, humans split from a common ancestor shared with chimpanzees. The first humans, the Australopithecines or contemporary species, stood upright on two legs and came down from the trees, expanding their habitat into competitive terrestrial environments. At this stage, however, their legs were short and their brains still small. For the next 4 million years, humans continued their evolution within Africa.

The species to which we all belong, Homo sapience, evolved from archaic Homo in Africa after 200,000 years ago. Repeated technological inventions derived from H. sapiens' complex cognitive abilities and enabled the conquest of cold , dry and other harsh environments, leading to rapid expansion across the globe. In this section you can experience for yourself the history of H. sapiens prior to the rise of urban civilization.

Humans moved from Africa into Eurasia some 1.8 million years ago, but for a long time their distribution was limited to low and middle latitudes. In contrast, the dispersals of Homo sapiens that began in earnest 500,000 years ago were vastly different in scale and speed. How did our ancestors adapt to the different environments they found around the world?

Large expanses of ocean stretch between the land areas of Eurasia and  Oceania. About 500,000 years ago, Homo sapiens began crossing this ocean, most likely in raft-like boats. Maritime technology become more advanced over time and distant ocean voyaging technology was developed, enabling humans to expand 13,000 km across the whole of the South Pacific.

Around 40,000 years ago, when there was a temporary easing in the glacial climate, the area of settlement of Homo sapiens began to expand into northern regions that had previously not been inhabited by humans. Despite later deteriorations in climate, humans reached the depths of Siberia and, several millennia before 10,000 years ago, some crossed via Alaska into the America.

The American continent stretches north to south for 14,000 km through a wide range of natural environments, from the poles to the plains, from deserts to high mountains, and from boreal and temperate to tropical rain forests. These diverse environments seem to have posed few problems, however, for the first humans who entered North America prior to 10,000 years ago and who quickly expanded down to the bottom of South America.