Earth was born as a result of repeated asteroid collisions, the moon
created by a single giant impact event. Then, Earth's size attracted
huge meteorites, which slammed into it, causing super-high-temperature
rock vapour to cover the entire surface and evaporate all ocean water.
The earliest life-forms survived such infernal events by escaping deep
into the ground, miraculously emerging again and again. The Earth has
gone through innumerable catastrophic events, and life has survived by
acquiring new abilities to live through each crisis. Humans are part of
the grand history of life's evolution, which has been closely
intertwined with repeated cataclysmic events. Why was Earth able to
evolve into a liveable planet, but not Mars, which failed to hold its
atmosphere and oceans? Find out in the opening episode of the series.
Snow Ball Earth
The hypothesis "Snowball Earth" explains that ice ages, caused by the earliest life-forms, resulted in an evolutional leap ? larger-sized life. Atmospheric methane created by microbes initially kept the Earth warm. As microbes that produce oxygen (photosynthesizers) emerged, atmospheric methane was lowered. As a result Earth cooled and the primeval ocean froze to 1,000 meters deep. Most life may have become extinct during a long period of intense glaciation believed to have lasted several million years. However, some life survived, perhaps in puddles created near volcanic craters. Carbon dioxide accumulated in the air because thick ice sheets prevented ocean water from absorbing it. A green-house effect, created by carbon dioxide, finally melted the ice and fed photosynthesizers. Hyper hurricanes raged after the great meltdown and stirred up the ocean water, creating an ideal condition for life to prosper and develop collagen. Using collagen, life-forms were able to build larger bodies, and a variety of creatures, called Ediacara biota, emerged for the first time.
New Frontiers tells the story of how "continental drift" brought our ancestors onto the land. Four hundred million years ago, shallow seas around primeval continents were home to most life-forms. The size of the shallow seas decreased as the supercontinents, like Laurasia formed. Some species found freshwater homes inland to escape from a struggle for survival in the ocean. Continental drift caused the formation of huge mountains. The peaks blocked atmospheric currents, creating clouds that rain. The rain gathered to make rivers at the foot of the mountains. Our ancestor, Eustenopteron, came to live in those rivers at the foot of the Caledonian mountains, where primeval trees called Archaeopteris developed a mild and stable environment. The leaves of Archaeopteris supplied nutrients for life, but caused oxygen depletion during dry seasons. Eustenopteron and other fish species developed proto-lungs to survive in low oxygen. Huge predators appeared as well. Acanthostega sought to hide from predators in Archaeopteris branches that piled in water near riverbanks. They made their way forward by pushing the branches aside, thus developing a forefoot. After several million years, Pederpes finally made the first step onto the land.
Extinction and Rebirth
Life on Earth has experienced five great extinctions. The worst occurred about 250 million years ago during the Permian period. Our ancestors at that time were mammal-like reptiles. The super-plume theory attempts to explain this extinction with a gigantic vertical flow of magma in Earth's mantle. About 300 million years ago, under the ocean trenches around the supercontinent Pangea, the end parts of the ocean plates dropped toward the core, creating a reactionary upward magmatic current that caused violent volcanic eruptions. A huge basaltic plateau in Siberia is believed to be evidence of this super-plume. According to the theory, carbon dioxide from volcanoes warmed Earth, causing methane-hydrate long frozen under the ocean floor to melt. Enormous amounts of methane were released into the air, accelerating global warming. The methane also depleted atmospheric oxygen, devastating plants. The oxygen depletion continued for about 100 million years. Reptiles adapted best by developing a unique air-sac system. Mammals were also forced to devise a way to adapt, so they evolved to nurture babies in a womb in order to send them adequate oxygen. Mammals also devised a diaphragm for efficient respiration. This caused them to lose the lower half of their ribs, but as a result they could lie on the ground while twisting their waist. Now they were capable of breast-feeding.
Survival Of The Fittest
Over its more than 4-billion-year history, Earth has been home to repeated violent climatic changes, which have caused mass extinctions. And yet, life has survived. These same catastrophes that devastated life on Earth also helped bring about its evolution.