Three and a half billion years ago, Earth's atmosphere contained almost no free oxygen. Instead, it consisted mainly of carbon dioxide, perhaps as much as 100 times more carbon dioxide than contained in today's atmosphere.
Oxygen, although always present in compounds in Earth's interior, atmosphere, and oceans, did not begin to accumulate in the atmosphere as oxygen gas (O2) until well into the planet's history.
The results suggest that the initial buildup of oxygen in the atmosphere was relatively rapid. Since its first appearance 2.33 billion years ago, oxygen accumulated in high enough concentrations to have a weathering effect on rocks just 10 million years later.
Earth's transition to permanently hosting an oxygenated atmosphere was a halting process that took 100 million years longer than previously believed, according to a new study. When Earth first formed 4.5 billion years ago, the atmosphere contained almost no oxygen.
With an environment devoid of oxygen and high in methane, for much of its history Earth would not have been a welcoming place for animals. The earliest life forms we know of were microscopic organisms (microbes) that left signals of their presence in rocks about 3.7 billion years old.
Life, as far as we know, has existed on Earth for about 3.5 billion years. At some point early on in that history, living things evolved the ability to breathe oxygen. “Life respired other things, like nitrous oxide, before oxygen.”
Some scientists estimate that 'life' began on our planet as early as four billion years ago. And the first living things were simple, single-celled, micro-organisms called prokaryotes (they lacked a cell membrane and a cell nucleus).
The dinosaurs apparently breathed air that was much richer in oxygen than our air and lived in forests and grasslands that were far more combustible than ours. The metabolisms evolved to live is such an atmosphere might be radically different from ours.
All of these individual molecules are constantly rearranged and recycled through biochemical and geochemical processes, so you aren't breathing in the exact same gas molecules that dinosaurs and Julius Caesar once breathed.
When the earth was formed around 4.5 billion years ago, it had vastly different conditions. At that time, the earth had a reducing atmosphere, consisting of carbon dioxide, methane and water vapor, as opposed to the present-day atmosphere that consists primarily of nitrogen and oxygen.
Three and a half billion years ago, Earth's atmosphere contained almost no free oxygen. Instead, it consisted mainly of carbon dioxide, perhaps as much as 100 times more carbon dioxide than contained in today's atmosphere.
A long time ago, before humans, dinosaurs, plants, or even bacteria, Earth's air had no oxygen. If we could time travel to that period, we would need space suits to breathe. Scientists think the air was mostly made out of volcanic gases like carbon dioxide.
Earth's oxygen supply originated with cyanobacteria, tiny water-dwelling organisms that survive by photosynthesis. In that process, the bacteria convert carbon dioxide and water into organic carbon and free oxygen.
Dinosaur skeletons captured in stunning detail
They found that oxygen levels in the atmosphere jumped from around 15 per cent to around 19 per cent. Today's atmosphere has about 21 per cent oxygen.
For Earth's first 2 billion years, no oxygen existed in the atmosphere. Low levels of oxygen first appeared when cyanobacteria, also called blue-green algae, began releasing oxygen as a byproduct of photosynthesis. Then, about 2.4 billion years ago, Earth underwent the Great Oxidation Event.
In just five seconds without the oxygen, the world would look completely different. Without oxygen, our houses, dams, tall building and any structure made out of concrete would collapse instantly. Oxygen is also important for the concrete.
If we used a time machine to travel back to a prehistoric period, the earliest we could survive would be the Cambrian (around 541 million years ago). Any earlier than that and there wouldn't have been enough oxygen in the air to breathe.
Between 850 and 600 million years ago, oxygen concentrations increased steadily from 2 to about 10 per cent: still not enough for humans to survive on. Fast forward to 400 million years ago and you could just about breathe but might feel dizzy and confused on about 16 per cent oxygen.
No! After the dinosaurs died out, nearly 65 million years passed before people appeared on Earth. However, small mammals (including shrew-sized primates) were alive at the time of the dinosaurs.
Earth's early atmosphere has seen two major spikes in oxygen concentration — one roughly 2.3 billion years ago, dubbed the 'great oxygenation event', and a second 800 million years ago. The evolution of complex life only took off after this second peak.
The beginning of the age of dinosaurs, about 215 million years ago, corresponded with an increase in atmospheric oxygen from 15 percent to 19 percent. The current atmosphere has about 21 percent oxygen so some of those early dinosaurs from the Triassic would likely be plenty comfortable running around today.
There is such thing as too much oxygen
Micro-organisms exhausted the nutrients they needed to create oxygen, which knocked the Earth's atmosphere off-kilter. This led to an “enormous drop” in the biosphere – the amount of life on Earth. Scientists weren't sure just how drastic the drop was until now.
Adam is the name given in Genesis 1-5 to the first human. Beyond its use as the name of the first man, adam is also used in the Bible as a pronoun, individually as "a human" and in a collective sense as "mankind".
The first human ancestors appeared between five million and seven million years ago, probably when some apelike creatures in Africa began to walk habitually on two legs. They were flaking crude stone tools by 2.5 million years ago. Then some of them spread from Africa into Asia and Europe after two million years ago.
Mineral-laden water emerging from a hydrothermal vent on the Niua underwater volcano in the Lau Basin, southwest Pacific Ocean. The microorganisms that live near such plumes have led some scientists to suggest them as the birthplaces of Earth's first life forms.