One of the big mysteries in the history of life is why it took so long time for complex organisms to evolve. The Earth is 4.5 billion years old and fossils of the first motile animals are ca. 555 million years old. It took 3,945,000,000 years or almost 4 million millennia before evolution in mostly microbial ecosystems evolved organisms with a capacity to swim and hunt in water. Surely, this had to happen before land could be conquered by plants and motile animals.
In a new study published in Nature, Jochen Brocks and colleagues find molecular biomarker evidence that algae first became abundant in the oceans between 659 and 645 million years ago and that smaller cyanobacteria dominated the marine realm before them. As a result, natural selection would favour larger heterotrophic organisms that could digest the larger food packages and biological evolution was now directed towards larger organisms. This may well have kickstarted evolution towards bigger organisms on Earth.
Larger red algae evolved more than 1600 million years ago, but according to the new study, most phytoplankton remained small more than 1000 million years later.
The question is now: why did it take so long for larger algae to become abundant in the oceans?
Brock and colleagues suggest that oceans were depleted in the nutrient, phosphate, which larger algae require in greater abundance than cyanobacteria. The melting of the Snowball Earth event 645 million years ago injected nutrients into the oceans that allowed algae – for the first time – to proliferate in the oceans (also delayed by an extremely warm climate immediately following the meltdown of the Snowball Earth).
While this explanation is not generally accepted, the new molecular biomarker data convincingly show that the marine ecosystems and the trophic chains changed from bottom up at the time animals evolved or shortly before.
Videnskab.dk have published an appetising article about this (in Danish).