New research suggests that complex life might have evolved on Earth must earlier than scientists thought.

It has been accepted by scientists for many years that the evolution of complex life is an incredibly rare event that can only occur once every 4.5 billion years. However, new research might completely change human understanding of complex life form evolution. Researchers at the University of Washington have suggested that it is entirely possibly that complex life forms evolved and died on this planet before the current lineage even began. Essentially, this could mean that other beings inhabited Earth billions of years ago which were completely different from all life on the planet today.

The planet Earth is believed to be 4.5 billion years old. Approximately 3.7 billion years ago, two of three kingdoms of life began to emerge – bacteria and archaea. These two kingdoms of single-celled organisms thrived on the planet for billions of years until the third kingdom of life – eukaryotes – began to emerge. The eukaryote family tree comprised all complex organisms on this planet including plants, fungi, protists, birds and mammals.

there was enough oxygen in Earth’s atmosphere between 2.4 and 2 billion years ago

Scientists are not entirely sure how this particular kingdom came into being but the most popular theory is that an ancient bacteria ingested an archaea and the two came to develop a symbiotic relationship. The archaea would eventually become the mitochondria visible in the cells of complex organisms today.

Researchers also stress that complex life could have only come about when there was an explosion of oxygen in the Earth’s atmosphere. It has always been presumed that oxygen was brought to Earth by the excretions of cynaobacteria over billions of years. The level of oxygen in the Earth’s atmosphere finally reached the levels required to support complex life forms approximately 1.6 billion years ago. At least, this is what scientists believed to be true. However, this new study from the University of Washington that this might not be entirely correct.

The researchers, who have been analysing the layers in ancient deposits of selenium, discovered that there was actually enough oxygen in the Earth’s atmosphere between 2.4 and 2 billion years ago to support complex life. Following that brief time period, the oxygen levels on the planet dropped off suddenly. According to the researchers, the reason for the sudden influx of oxygen and its disappearance can only be speculated on at this point in time.

What this discovery means is that the conditions to support complex life forms were evident on the planet Earth long before the emergence of the eukaryote kingdom. The researchers say that despite the fact that no fossilised multi-cell life predating the development of the current lineage has ever been found does not mean that it does not exist. “There is fossil evidence of complex cells that go back maybe 1.75 billion years, “said astrobiologist Roger Buick. “But the oldest fossil is not necessarily the oldest one that ever lived – because the chances of getting preserved as a fossil are pretty low. This research shows that there was enough oxygen in the environment to have allowed complex cells to have evolved, and to have become ecologically important, before there was fossil evidence. That doesn’t mean that they did – but they could have.”

While it is entirely possible that scientists will never really discover proof for the emergence of life prior to the eukaryote kingdom, this discovery is still potentially groundbreaking. The researchers have proven that the conditions which allowed for the development of complex life forms on this planet are not as rare as scientists have previously supposed. This means that the chance that life has developed on planets other than the planet Earth seems significantly more likely than was once thought.

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