Life on Earth

A billion years after the formation of the Earth, simple blue-green algae (prokaryotic cells) developed and were the only life on the planet for another 2 billion years. Examples of these algae cells were discovered in northwestern Australia in a silica-rich sedimentary rock called chert dated at 3.77 billion years old. A billion and a half years ago, the cells then evolved nuclei, and became eukaryotic cells. Another billion years passed before multicellular living things evolved, and all the rest of the profusion of life has developed in the last 500,000 years, just one-ninth of the age of the Earth.

Once life developed the ability to photosynthesize, cells began to give off oxygen as they created energy. During the Proterozoic Eon, from 2.5 billion to 542 million years ago, the oxygen content of the atmosphere rose from about 1 to 10 percent because of this input. Not until the Devonian Period, about 400 million years ago, did the oxygen content of the atmosphere reach its current 21 percent. As oxygen increased in the atmosphere thanks to early life, fish developed in the Cambrian Period, about 500 million years ago. Land plants developed in the mid-Silurian Period, about 420 million years ago. Reptiles developed in the early Pennsylvanian Epoch, about 360 million years ago, and amphibians developed in the Devonian Period, about 340 million years ago.The earliest primates did not appear until about 57 million years ago.

Though there has been an overall increase of the numbers and diversity of species over the age of the Earth, life on Earth has gone through violent cycles, involving periods of development and diversity punctuated by extinctions.The greatest of the extinctions occurred 250 million years ago, at the boundary between the Permian and Triassic periods. At that moment in time, 95 percent of marine species and 70 percent of land-based species

The Manicouagan impact crater in eastern Canada has been so eroded over time that only a circular depression, filled here with frozen water, remains. (Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, eol.jsc.nasa.gov, image number ISS06-E-47702)

existing on Earth were wiped out. At the same time, the geologic rock record shows us, there was both a huge volcanic eruption in Siberia, a great fall in sea level, and changes in oceanic chemistry.

The interactions and causes of these simultaneous events are hotly debated in the scientific community. Some think a plume of hot material from the lowest mantle rose, melted, and erupted, changing climate violently enough to cause the extinctions, while others suggest that a huge meteorite impact both changed the climate and caused the volcanic eruptions. Giant impacts are the only known natural disaster that can wipe out all life on Earth. The image here shows the Manicouagan impact crater in eastern Canada, an impact large enough to have caused significant climatic disruption, but not enough to cause a global extinction. This photo of Manicouagan was taken from the International Space Station and shows the water filling the outer rim of the original crater has frozen in winter. The Manicouagan crater is an excellent example of an ancient crater partly erased by erosion until it consists of nothing but a circular lake. This image shows the reservoir now filling the crater frozen in winter.

Though the causes and interactions remain incompletely understood, study of extinctions does show that life on Earth is fragile. Extinctions have occurred regularly throughout Earth history and all of life is vulnerable to the violent actions of this mobile planet and to the effects of giant meteorite impacts. A list of the five largest extinctions in Earth history is given in the table on page 108. Many more of smaller magnitudes have occurred. In relatively small ways, earthquakes form hazards:The most deadly earthquake in recorded history took place in China's Shaanxi province in 1556, killing 800,000 people. Volcanoes, on the other hand, can change the entire planet's climate: 73,500 years ago a volcanic supereruption occurred in Sumatra, filling the atmosphere with sulfur, ash, and carbon dioxide, blocking the Sun's energy to such an extent that plants couldn't sufficiently photo-synthesize, and the global ecosystem almost collapsed. Studies of human DNA suggest that there were only a few thousand early human survivors of that catastrophe. Much more recently, in 1815, Mount Tambora, on the Indonesian island of Sumbawa, erupted with an explosion that was heard 875 miles (1,400 km) away.Twelve thousand Indonesians died immediately from the eruption, and another 80,000 died later as a result of famine, as crops were wiped out. Another volcanic winter was caused by the ash cloud that enveloped the Earth, and in England and the United States, frosts occurred all summer.

The perennial question of why life developed on Earth has no clear answer. Key factors were present, including a moderate temperature and the presence of water, but perhaps mankind thinks of these as necessities for life simply because they are necessities for the life that has developed on Earth. Though many scientists hope and think that primitive life may have developed on Mars at some point in the past, no evidence for such life has been found. The only place in the universe that life is known to exist is here on Earth.

Reseachers have developed a number of interwoven and complex theories for how the solar system formed.The cloud of material orbiting the young Sun is supposed to have been composed of different materials depending on the distance from the Sun.The closest materials were rocky and metallic, able to withstand the intense heat near the new star. At a distance between where Mars and Jupiter now orbit, temperatures fell sufficiently to allow more volatile compounds to condense into ices. The point where ices could condense is often referred to as the snow line. As the planets accreted out of smaller

THE MOST EXTREME EXTINCTIONS IN EARTH HISTORY

Name

Time

Effect

Cretaceous-Tertiary

65 million years

possibly caused by a meteorite impact; 16 percent

before present

of marine families, 47 percent of marine genera,

and 18 percent of land vertebrate families, including

the dinosaurs, went extinct

End-Triassic

199 to 214 million

possibly caused by volcanism at the opening of

years before present

the Atlantic ocean; 22 percent of marine families,

52 percent of marine genera went extinct

Permian-Triassic

251 million years

possibly caused by the Siberian flood basalts or

before present

by a giant impact; 95 percent of marine species,

including the trilobites, and 70 percent of land

species went extinct

Late Devonian

364 million years

22 percent of marine families and 57 percent of

before present

marine genera went extinct

Ordovician-Silurian

439 million years

25 percent of marine families and 60 percent of

before present

marine genera went extinct

pieces of material and increasingly attracted new material to themselves by their increasing gravitational fields, they came to be made of averages of the compositions available to them at their distance from the Sun.The inner, terrestrial planets are therefore made of rocky and metallic material, and the planets outside the snow line contain a large fraction of ices. In this way scientists made a theory for the formation of planetary systems that depended exclusively on one example only: This solar system.There was no other example for comparison.

When extrasolar planets began to be discovered, the overarching theories for the formation of planetary systems were exploded. The largest of planets are the easiest to discover, since they influence their stars the most, by pulling the star with their gravity as they orbit so that the star wobbles, or by blocking a portion of the star's light if they pass between the star and Earth. Scientists began to discover a large number of giant planets orbiting distant stars. More than 100

extrasolar planets have been discovered; their masses range from one-tenth to 13 Jupiter masses (at about 15 Jupiter masses, the object will have the gravity necessary to become a star). All of the extrasolar planets now known orbit their stars more closely than 5 AU. In this solar system, 5 AU reaches just to Jupiter. The largest number of extrasolar planets orbit much closer to their stars, and the number of planets drops off steeply toward orbits of 5 AU. Out there was a large population of planetary systems that looked and behaved nothing like this solar system. This solar system is the only known example with giant planets orbiting far away, and now there are more than 100 that are completely different. What are the general laws for forming planetary systems? Scientists obviously need to think again! These giant extrasolar planets are thought to have formed further away from their stars; up close to the star, there is not enough material to make a giant planet. Did they form a core first, and then attract other materials gravitationally, as the Earth probably did, or did they collapse out of the early nebula like a new star does? If they formed farther away from the star, where there was more material to make a giant planet, why did they migrate inward to a tiny, close orbit? Why have the giant planets Jupiter and Saturn not done the same?

By studying extrasolar planets scientists hope to discover more about the Earth and how this solar system formed, and also to search for likely places to find life elsewhere in the universe.They hope to get a sense for the inventory of solar systems in the universe. Are there a continuum of planet sizes between small terrestrial planets like Earth and the giant planets that can now be detected orbiting distant stars, or are there really two populations of planet sizes? Where do ocean planets fit in this inventory? What is the inventory of solar systems? This is the big, central question: Since life requires water (if there are kinds of life that do not, they have not been discovered or imagined), scientists want to find planets with oceans. If there are other planets with oceans, can humans find life elsewhere in the universe?

Extrasolar planets can be detected by looking for stars that wobble from the gravitational pull of large planets that are circling them. More rarely, the planets pass between the star and Earth in their orbit, called a transit. Transiting takes a particular range of orientations and orbits. Observers could be looking down on the planetary system, for example, and the planet would appear to orbit around its

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