How old is the world evolution

Thereafter, the number of impacts appeared to have quickly decreased. This observation rejuvenated the theory of accretion postulated by Otto Schmidt. The Russian geophysicist had suggested in that planets grew in size gradually, step by step. According to Schmidt, cosmic dust lumped together to form particulates, particulates became gravel, gravel became small balls, then big balls, then tiny planets, or planetesimals, and, nally, dust became the size of the moon.

As the planetesimals became larger, their numbers decreased. Consequently, the number of collisions between planetesimals, or meteorites, decreased. Fewer items available for accretion meant that it took a long time to build up a large planet. A calculation made by George W. Wetherill of the Carnegie Institution of Washington suggests that about million years could pass between the formation of an object measuring 10 kilometers in diameter and an object the size of Earth. The process of accretion had significant thermal consequences for Earth, consequences that forcefully directed its evolution.

Large bodies slamming into the planet produced immense heat in its interior, melting the cosmic dust found there. The resulting furnace--situated some to kilometers underground and called a magma ocean--was active for millions of years, giving rise to volcanic eruptions.

When Earth was young, heat at the surface caused by volcanism and lava ows from the interior was intensified by the constant bombardment of huge objects, some of them perhaps the size of the moon or even Mars. No life was possible during this period. Beyond clarifying that Earth had formed through accretion, the Apollo program compelled scientists to try to reconstruct the subsequent temporal and physical development of the early Earth.

This undertaking had been considered impossible by founders of geology, including Charles Lyell, to whom the following phrase is attributed: No vestige of a beginning, no prospect for an end. This statement conveys the idea that the young Earth could not be re-created, because its remnants were destroyed by its very activity. But the development of isotope geology in the s had rendered this view obsolete. Their imaginations red by Apollo and the moon ndings, geochemists began to apply this technique to understand the evolution of Earth.

Dating rocks using so-called radioactive clocks allows geologists to work on old terrains that do not contain fossils. The hands of a radioactive clock are isotopes--atoms of the same element that have different atomic weights--and geologic time is measured by the rate of decay of one isotope into another [see "The Earliest History of the Earth," by Derek York; Scientific American , January ].

Among the many clocks, those based on the decay of uranium into lead and of uranium into lead are special. Geochronologists can determine the age of samples by analyzing only the daughter product--in this case, lead--of the radioactive parent, uranium.

In the classic work of Claire C. Patterson of the California Institute of Technology used the uranium-lead clock to establish an age of 4. As Patterson argued, some meteorites were indeed formed about 4. But Earth continued to grow through the bombardment of planetesimals until some million to million years later. At that time This possibility had already been suggested by Bruce R. Doe and Robert E. Zartman of the U.

Geological Survey in Denver two decades ago and is in agreement with Wetherills estimates. The emergence of the continents came somewhat later. According to the theory of plate tectonics, these landmasses are the only part of Earth's crust that is not recycled and, consequently, destroyed during the geothermal cycle driven by the convection in the mantle.

Continents thus provide a form of memory because the record of early life can be read in their rocks. Geologic activity, however, including plate tectonics, erosion and metamorphism, has destroyed almost all the ancient rocks.

Very few fragments have survived this geologic machine. Nevertheless, in recent decades, several important nds have been made, again using isotope geochemistry. One group, led by Stephen Moorbath of the University of Oxford, discovered terrain in West Greenland that is between 3. In addition, Samuel A. Bowring of the Massachusetts Institute of Technology explored a small area in North America--the Acasta gneiss--that is thought to be 3.

Ultimately, a quest for the mineral zircon led other researchers to even more ancient terrain. Typically found in continental rocks, zircon is not dissolved during the process of erosion but is deposited in particle form in sediment. A few pieces of zircon can therefore survive for billions of years and can serve as a witness to Earths more ancient crust.

Lancelot, later at the University of Marseille and now at the University of Nmes, respectively, as well as with the efforts of Moorbath and Allgre. It was a group at the Australian National University in Canberra, directed by William Compston, that was nally successful. The team discovered zircons in western Australia that were between 4.

Zircons have been crucial not only for understanding the age of the continents but for determining when life rst appeared. The earliest fossils of undisputed age were found in Australia and South Africa. These relics of blue-green algae are about 3. Manfred Schidlowski of the Max Planck Institute for Chemistry in Mainz studied the Isua formation in West Greenland and argued that organic matter existed as long ago as 3.

Because most of the record of early life has been destroyed by geologic activity, we cannot say exactly when it rst appeared--perhaps it arose very quickly, maybe even 4. Stories from gases ONE OF THE MOST important aspects of the planet's evolution is the formation of the atmosphere, because it is this assemblage of gases that allowed life to crawl out of the oceans and to be sustained.

Researchers have hypothesized since the s that the terrestrial atmosphere was created by gases emerging from the interior of the planet. When a volcano spews gases, it is an example of the continuous outgassing, as it is called, of Earth. But scientists have questioned whether this process occurred suddenly--about 4. These gases--including helium, argon and xenon--have the peculiarity of being chemically inert, that is, they do not react in nature with other elements. Two of them are particularly important for atmospheric studies: argon and xenon.

Argon has three isotopes, of which argon 40 is created by the decay of potassium Xenon has nine, of which xenon has two different origins. Xenon arose as the result of nucleosynthesis before Earth and solar system were formed.

It was also created from the decay of radioactive iodine , which does not exist on Earth anymore. This form of iodine was present very early on but has died out since, and xenon has grown at its expense. Like most couples, both argon 40 and potassium 40 and xenon and iodine have stories to tell. They are excellent chronometers. Although the atmosphere was formed by the outgassing of the mantle, it does not contain any potassium 40 or iodine All argon 40 and xenon , formed in Earth and released, are found in the atmosphere today.

Xenon was expelled from the mantle and retained in the atmosphere; therefore, the atmosphere-mantle ratio of this element allows us to evaluate the age of differentiation. Argon and xenon trapped in the mantle evolved by the radioactive decay of potassium 40 and iodine Thus, if the total outgassing of the mantle occurred at the beginning of Earths formation, the atmosphere would not contain any argon 40 but would contain xenon The major challenge facing an investigator who wants to measure such ratios of decay is to obtain high concentrations of rare gases in mantle rocks because they are extremely limited.

Fortunately, a natural phenomenon occurs at mid-ocean ridges during which volcanic lava transfers some silicates from the mantle to the surface. The small amounts of gases trapped in mantle minerals rise with the melt to the surface and are concentrated in small vesicles in the outer glassy margin of lava ows. This process serves to concentrate the amounts of mantle gases by a factor of 10 4 or 10 5. Collecting these rocks by dredging the seaoor and then crushing them under vacuum in a sensitive mass spectrometer allows geochemists to determine the ratios of the isotopes in the mantle.

The results are quite surprising. Calculations of the ratios indicate that between 80 and 85 percent of the atmosphere was outgassed during Earths rst one million years; the rest was released slowly but constantly during the next 4. The composition of this primitive atmosphere was most certainly dominated by carbon dioxide, with nitrogen as the second most abundant gas. Trace amounts of methane, ammonia, sulfur dioxide and hydrochloric acid were also present, but there was no oxygen. Except for the presence of abundant water, the atmosphere was similar to that of Venus or Mars.

The details of the evolution of the original atmosphere are debated, particularly because we do not know how strong the sun was at that time. Some facts, however, are not disputed. Homo habilis in Africa , using stone tools for cleaving meat from bone.

Homo antecessor in western Europe Atapuerca , Spain , closely related to the last common ancestor of Neanderthals, Denisovans and modern humans. Homo sapiens enter Eurasia Greece : first of multiple dispersals out of Africa by humans with early modern traits, including globular braincase and descended larynx facilitating spoken language.

Hunter-gatherer nomads. Homo with mix of archaic-human and Neanderthal traits Nesher Ramla , Israel : stone-tool industry, cooking meat; cultural exchange with humans?

Eurasian Homo sapiens co-existing with Homo floresiensis soon extinct and Homo luzonensis , interbreeding with Neanderthals and Denisovans. Anatomically modern humans henceforth the only hominin. Agricultural farming and settlements. The volume of this impressive mountain is thus:. Now, imagine a stream that flows down the side of this mountain. Mountain streams carry silt and sand downwards—a key factor in erosion. All of these sediments came from higher up the mountain, which is constantly being eroded away.

To estimate how long a mountain might survive against erosion, consider a mountain with six principal streams. A typical stream might carry an average of one-tenth of a cubic meter of rock and soil a few shovels full per day off the mountain, though the actual amount would vary considerably from day to day.

Over a period of a year, the six streams might thus remove:. That means every year on the order of cubic meters of material, or about 20 dump trucks full of rock and soil, might be removed from a mountain by normal stream erosion. If the mountain streams remove about cubic meters per year, then the lifetime of the mountain can be estimated as the total volume of the mountain divided by the volume lost each year:.

This estimate is certainly rough and not directly applicable to any specific mountain. Nevertheless, a few hundred million years is but a small fraction of a few billion years. How can we say Earth is 4. The physical process of radioactive decay has provided Earth scientists, anthropologists, and evolutionary biologists with their most important method for determining the absolute age of rocks and other materials Dalrymple ; Dickin Trace amounts of isotopes of radioactive elements, including carbon, uranium, and dozens of others, are all around us—in rocks, in water, and in the air Table 1.

The rest of the uranium will have decayed to , atoms of other elements, ultimately to stable i. Wait another 4. Radiometric dating relies on the clock-like characteristics of radioactive decay. In one half-life, approximately half of a collection of radioactive atoms will decay.

Source: NCSE. The best-known radiometric dating method involves the isotope carbon, with a half life of 5, years. Every living organism takes in carbon during its lifetime. At this moment, your body is taking the carbon in your food and converting it to tissue, and the same is true of all other animals. Plants are taking in carbon dioxide from the air and turning it into roots, stems, and leaves.

But a certain small percentage of the carbon in your body and every other living thing—no more than one carbon atom in every trillion—is in the form of radioactive carbon As long as an organism is alive, the carbon in its tissues is constantly renewed in the same small, part-per-trillion proportion that is found in the general environment. All of the isotopes of carbon behave the same way chemically, so the proportions of carbon isotopes in the living tissue will be nearly the same everywhere, for all living things.

When an organism dies, however, it stops taking in carbon of any form. From the time of death, therefore, the carbon in the tissues is no longer replenished.

Like a ticking clock, carbon atoms transmute by radioactive decay to nitrogen, atom-by-atom, to form an ever-smaller percentage of the total carbon. Scientists can thus determine the approximate age of a piece of wood, hair, bone, or other object by carefully measuring the fraction of carbon that remains and comparing it to the amount of carbon that we assume was in that material when it was alive. If the material happens to be a piece of wood taken out of an Egyptian tomb, for example, we have a pretty good estimate of how old the artifact is and, by inference, when the tomb was built.

The result: the two independent techniques yield exactly the same dates for ancient fossil wood. Carbon dating often appears in the news in reports of ancient human artifacts. In a highly publicized discovery in , an ancient hunter was found frozen in the ice pack of the Italian Alps Fig. The technique provided similar age determinations for the tissues of the iceman, his clothing, and his implements Fowler Carbon dating revealed that he died about 5, years ago. Photo courtesy South Tyrol Museum of Archaeology, www.

Carbon dating has been instrumental in mapping human history over the last several tens of thousands of years. When an object is more than about 50, years old, however, the amount of carbon left in it is so small that this dating method cannot be used. To date rocks and minerals that are millions of years old, scientists must rely on similar techniques that use radioactive isotopes of much greater half-life Table 1.

Among the most widely used radiometric clocks in geology are those based on the decay of potassium half-life of 1. In these cases, geologists measure the total number of atoms of the radioactive parent and stable daughter elements to determine how many radioactive nuclei were present at the beginning. Thus, for example, if a rock originally formed a long time ago with a small amount of uranium atoms but no lead atoms, then the ratio of uranium-to-lead atoms today can provide an accurate geologic stop watch.

When you see geologic age estimates reported in scientific publications or in the news, chances are those values are derived from radiometric dating techniques. In the case of the early settlement of North America, for example, carbon-rich campfire remains and associated artifacts point to a human presence by about 13, years ago. Much older events in the history of life, some stretching back billions of years, are often based on potassium dating. This technique works well because fossils are almost always preserved in layers of sediments, which also record periodic volcanic ash falls as thin horizons.

Volcanic ash is rich in potassium-bearing minerals, so each ash fall provides a unique time marker in a sedimentary sequence. The rise of humans about 2. Paleontologists rely on radiometric dating to determine the ages of fossils, such as this million-year-old trilobite, Ameura major , from near Kansas City, Kansas.

Photo courtesy Hazen Collection, Smithsonian Institution. The oldest known rocks, including basalt and other igneous formations, solidified from incandescent red-hot melts.

These durable samples from the moon and meteorites are typically poor in potassium, but fortunately, they incorporate small amounts of uranium and other radioactive isotopes. As soon as these molten rocks cool and harden, their radioactive elements are locked into place and begin to decay.

The most ancient of these samples are several types of meteorites, in which slightly more than half of the original uranium has decayed to lead. These primordial space rocks, the leftovers from the formation of Earth and other planets, yield an age of about 4. The oldest known moon rocks, at about 4. Only a few uranium-rich, sand-sized grains of the hardy mineral zircon, some as old as 4.

Nevertheless, uranium-bearing rocks, on every continent provide a detailed chronology of the early Earth Hazen et al. The oldest Earth rocks, at about four billion years, point to the early origins of continents. Rocks from almost 3. Distinctive uranium-rich sedimentary formations and layered deposits of iron oxides from about 2. Indeed, every stage of Earth history has been dated with exquisite accuracy and precision thanks to radiometric techniques.

Stromatolites, such as this 2. Radiometric methods provide an accurate approach to dating such ancient sediments. Photo courtesy of Dominic Papineau. Overwhelming observational evidence confirms that Earth history is the story of the co-evolving geospheres and biospheres: Life has changed continuously over the course of Earth history. As the work of Eugenie Scott has so forcefully defended, Earth must be billions of years old Scott However, such a conclusion is at odds with the doctrine of many Christian fundamentalists, who believe in the literal Biblical chronology of a universe no more than about 10, years old.

How can science respond to such adamant claims? The testimony of the rocks is unambiguous: an enormous body of observational evidence points to the reality of deep time. Annual ice and rock layerings reveal a million years of Earth history. Geologic rates of mountain building, erosion and plate tectonics demand hundreds of millions of years. Radiometric dating pushes the history back billions of years. And when these techniques overlap, their independent estimates of the timing of ancient events are internally consistent.

And then, remarkably, he proceeds to describe how God created everything 10, years ago to look much older!