Life on Earth evolved almost as soon as the planet cooled, but animals didn't arrive until more than 3 billion years later. The discovery of pigments from 1.1 billion years ago may have explained this great delay. The site is astonishing because it was laid down 600 million years earlier than the previously oldest known pigments.
In the course of her PhD Dr Nur Gueneli of the Australian National University explored some black shales from the Taoudeni Basin, Mauritania. Astonishingly, among the black she found some pink highlights, something never before seen for rocks this old.
“The bright pink pigments are the molecular fossils of chlorophyll that were produced by ancient photosynthetic organisms inhabiting an ancient ocean that has long since vanished,” Gueneli said in a statement.
Gueneli crushed the rocks for analysis and found they were made almost entirely from cyanobacteria. This is despite molecular clocks placing the appearance of the first algal plants much earlier, 1.5-1.9 billion years ago.
Cyanobacteria's dominance, and the rareness of algae, made conditions unsuitable for animals, said Dr Jochen Brocks of the Australian National University, senior author of the paper published in Proceedings of the National Academy of Sciences. “Algae, although still microscopic, are a thousand times larger in volume than cyanobacteria, and are a much richer food source.” Even the smallest animals would have got little sustenance from such tiny particles.
Brocks told IFLScience the pink color that alerted Gueneli to the deposit would not have been seen at the time. “One-point-one billion years ago these were blue-green chlorophylls, with a ring structure around a magnesium atom,” he said. “In anoxic sediments, the ring structure survived, but the magnesium was replaced with nickel or vanadium, making the molecules blood red or pink.”
While the paper was in production another pigment-preserving sediment was found dating to 550 million years ago was found, slightly extending the previous record. However, Brocks doubts much will be found to otherwise bridge the gap. “This is just a very lucky sample...rocks that preserve marine pigments are very rare,” he said. We don't know how typical this location was of the era, but other sources suggest algae didn't become common until 650 million years ago.
Although uncertain why cyanobacteria out-competed algae for so long, before being suddenly overtaken, Brocks told IFLScience the oceans of the day were probably very low in phosphorus. The high surface area to volume of the bacteria allowed them to capture the little that was going, while it is speculated the larger algae couldn't compete. When oxygen levels rose, more phosphorus was released. With nutrients more available, being too large for protozoans to eat gave algae an advantage, one that eventually served our animal ancestors well.