About 2.7 billion years ago, the primordial seas already hosted the first photosynthetic microbes, the blue-green algae that took in carbon dioxide and released oxygen into the air. But they were outnumbered by methane-producing bacteria called methanogens [that] thrived in nickel-rich seas. The high amounts of methane that this early life pumped into the environment prevented oxygen accumulation in the atmosphere because the methane reacted with any oxygen, creating carbon dioxide and water [Science News], according to one theory. Now, a group of researchers say they’ve found the trigger that allowed oxygen to build up, and therefore allowed for a profusion of oxygen-breathing life.
The secret was the concentrations of the metal nickel, according to the new study, published in Nature. The scientists found that by analysing a type of sedimentary rock known as banded-iron formations they could monitor levels of nickel in the oceans of the early Earth dating as far back as 3.8 billion years ago. They found there was a marked fall in nickel between 2.7 billion and 2.5 billion years ago [The Independent]. That stretch of time correlates with what researchers call the Great Oxidation Event, when oxygen began to take hold in the atmosphere.
The chemical traces of primitive sea sponges have been found in rocks dating from 635 million years ago, and researchers say it’s the earliest definitive evidence of animal life yet. The finding pushes the origin of animal life back to a truly inhospitable epoch known as the Cryogenian, when glaciers rolled across the planet and ice may have frozen over the seas, and when the deep ocean didn’t yet contain oxygen. Yet somehow, early sponges known as demosponges thrived in that environment. Says lead researcher Gordon Love: “We’re not saying we captured the first animal; we’re saying they’re an early animal phylum and we’re capturing them when their biomass was significant” [BBC News].
Researchers can usually determine the presence of ancient life in rock strata by looking for the fossilised remains of skeletons or the hardened record of the creatures’ movements, such as their footprints or crawl marks. But for organisms deep in geological history that were extremely small and soft bodied, scientists have had to develop novel techniques to uncover their existence [BBC News]. Love says the early demosponges were only a few millimeters in diameter, but they still left their mark in the form of “chemical fossils.” In rocks from an Oman oil field, researchers detected a chemical produced when the cell membranes of sponges break down.
A new genetic analysis has shaken up the tree of life, dispelling the common assumption that sea sponges or comb jellies are the original ancestors of all animals. That original animal, also referred to as the “ur-animal,” is thought to have given rise to both the “lower” animals (Cnidaria), such as coral and jellyfish, and “higher” animals (Bilataria), such as insects and humans. Based on the new study, researchers are now putting forth a new classification, which would place sponges among the “lower” animals, leaving an open spot for the original animal. “It’s a question that has plagued animal biologists for a couple hundred years: What could be the mother of all animals?” said [researcher] Rob DeSalle… “We’ve turned it upside down” [Wired Science].
Taxonomy has come a long way since the Linnaean system, based largely on comparative anatomy, was introduced in 1735. The research team fed morphological data on the appearance of animals from 24 taxa together with genetic information into a computer program that assessed similarities and differences to generate a phylogenetic tree of life [Nature News]. The results placed placozoans, a simple amoeba-like but multi-celled organism, as a more ancient animal than even the sea sponges. Yet, at the same time, the data suggests that placozoans is not the last common ancestor of all animals because they are not directly related to the more complex Bilataria. “It fits in with what you might think is the most basal animal. It’s only got three cell layers and four cell types. Its motility is primitive. It lives in warm oceans. It’s got all the earmarks of the thing that gave rise to all animal life,” said DeSalle [of placozoans]. “But that’s not what the results show. And though placozoa is the ur-cousin of complicated life, we still don’t know the ur-mother” [Wired Science].
On the seafloor near the Bahamas, researchers have discovered a single-celled organism about the size of a grape, and they say the unusual organism raises interesting questions about the evolution of complex, multicellular animals. The oversized protists were found at the end of long, linear tracks that appear to have been made by the slowly rolling amoebas; lead researcher Mikhail Matz says the tracks resemble fossilized impressions from over 1 billion years ago, which scientists had assumed were made by multicellular worms. “We were looking for pretty animals that have eyes, are coloured, or glow in the dark; instead, the most interesting find was the organism that was blind, brainless, and completely covered in mud,” he said [BBC News].
The origin of multicellular life has been shrouded in mystery, because few animals fossils have been found that predate the beginning of the Cambrian Period around 542 million years ago. Some researchers point to rare Precambrian “trace fossils” – such as slither prints left in ancient sea bottoms – as evidence for complex animal life predating the Cambrian. The oldest of these trace fossils yet found are 1.8 billion years old, about three times older than any animal in the fossil record [The Scientist]. However, the new tracks raise another possibility: that the ancient traces were created by large single-celled organisms.
In the middle of the Australian outback along a mountain chain called the Flinders Ranges, researchers have discovered a 650 million year old reef that was once underwater. Researchers say the tiny fossils they’ve already found in the ancient reef may be the earliest examples of multicellular organisms ever found, and may answer questions about how animal life evolved.
Researcher Malcolm Wallace explains that the oldest-known animal fossils are 570 million years old. The reef in the Flinders Ranges is 80 million years older than that and was, he said, “the right age to capture the precursors to animals” [The Times]. The first fossils discovered in the reef appear to be sponge-like multicellular organisms that resemble tiny cauliflowers, measuring less than an inch in diameter, but Wallace cautions that the creatures haven’t been thoroughly studied yet. The reef’s discovery was announced at a meeting of the Geological Society of Australia this week.