The history of life on Earth has been uncovered through research into what is known as the fossil record. The fossil record assists paleontologists, archaeologists and geologists to place important historic events and species in the correct geologic era. Fossils are either dated through the Law of Superposition – meaning that in undisturbed rock layers, the bottom layers are older than the top layers – or through carbon dating – using the radiative decay of carbon-14 isotopes over thousands of years to determine the age of rocks and fossils.
The Cambrian Explosion
With every new fossil discovered, humans are able to better understand the environment at a particular time and piece together the evolutionary tree. Most major groups of animals appear in the fossil record during the Cambrian Explosion, about 541 million years ago. In the Cambrian Explosion, many forms of complex animal life appeared suddenly in the rock record, with no evolutionary precursors. These complex multicellular organisms discovered in the rocks are known as metazoans, and Cambrian metazoan fossils include clams, snails, crabs, sponges, worms, sea urchins, starfish and other complex invertebrates. Making the Cambrian Explosion even more notable is the fact that the rock layers below the Cambrian do not contain any complex invertebrates – this is known as Darwin’s Dilemma.
Could Life Be Older?
The oldest undisputed sponge fossils are mineralized pointy structures found in many types of sponges known as “spicules”. A team of researchers has discovered spicule fossils that dated back to just before the Cambrian Explosion, about 535 million years old, and published their findings in the Journal of the Geological Society.
According to a paper published in Nature, possible fossils of sponges discovered in north-western Canada could be 350 million years older than the Cambrian Explosion, pushing back the date of the earliest instances of complex life. This discovery is fueling debates, as paleontologists have long argued over when complex animal life first evolved on Earth. Some scientists are however not convinced that the sponge-like fossils found in Canada indicate an ancient sponge, debunking the claims that it could be the earliest discovered animal. It is possible that the fossils found belong to microbes, which can take on many different shapes and sizes that may resemble sponges. It is also possible that the patterns found on the rocks are due to historic crystal growth.
The fossils were collected from ancient microbial reefs preserved in the rocks of Canada’s Northwest Territories. About a billion years ago, this region of Canada – now defined by rugged mountains – was a prehistoric marine environment containing reefs built by cyanobacteria. When investigated under a microscope, the slices of rock contained three-dimensional structures and branching networks of crystalline tubes, resembling the internal structures of today’s sea sponges. The structure of the crystalline tubes also lines up with expected decay and fossilization patterns of spongin, the collagen protein responsible for a sponge’s structure, as well as resembling modern keratosan sponges.
Although the fossils were uncovered in the 1990s, Elizabeth Turner, a professor of carbonate sedimentology and invertebrate paleontology at Laurentian University in Canada, and the scientist who discovered them during her graduate studies, only published her results recently, when she saw studies describing similar structures in much younger rocks. These similar studies are also disputed for their validity, as it remains uncertain whether they show actual sponge fossils. Evidence does show that sponges possibly developed more than 700 million years ago, which is estimated using chemical compounds from sponges that are preserved in rock.
If the fossils do turn out to be sponges from 890 million years ago, it could help scientists nail down the early story of animal evolution, prior to the Cambrian Explosion. Unfortunately, it is easy to find structures resembling sponges in fossils, but it is more difficult to back them up with hard evidence to support the claims. It is not completely inconceivable that sponges pre-dated the Cambrian Explosion. Research into the ancestors of modern living animals measures the rate of mutation in DNA and proteins in living animals over time. Using this “molecular clock” it is estimated that the common ancestor of all animals that are alive today had evolved before the Cambrian Explosion, however probably not by as much as 350 million years.
A Hotly Debated Topic
Scientists believe that life on Earth appeared around 3.7 billion years ago, but that the first animals only appeared much later. Paleontologists are however divided on whether the absence of complex animal life prior to the Cambrian period is because animals that were alive then did not survive as fossils to present day, or whether the molecular clock estimations of animal origins are wrong and animals have been on Earth far prior to the Cambrian Explosion. Most paleontologists agree that it is possible sponges existed prior to the Cambrian Explosion, but it remains unresolved exactly how far back they lived.
If sponges were in fact alive 890 million years ago (350 million years before the Cambrian Explosion), it means that animals survived in very tough conditions, including living with low oxygen levels and “snowball Earth” phases, where the entire planet froze over during a series of ice ages. Sponges are multicellular organisms and require oxygen to metabolize. They also feed by extracting food particles from water pumped through their bodies. It is possible that sponges survived these trying low-oxygen and ice age conditions by living in cavities in the microbial reef, next to photosynthetic cyanobacteria, which release oxygen. The sponges may have also survived by feeding on organic ooze produced by the cyanobacteria.
It would not be surprising to discover that the earliest sponges – and perhaps the earliest life – were reef-dwellers, due to oxygen produced by the cyanobacteria building these reefs. All life on Earth has an ancestor, and it has always been predicted that the first evidence of complex animal life would be small and cryptic. Perhaps the discovery by Turner could be the very subtle clue towards the true age of complex animal life on Earth.