Two new studies shine light on how far evolution has come over the last half a billion years.
The first study, out of the Massachusetts Institute of Technology, determined that sea sponges were most likely the first animals on Earth, having been around for at least 640 million years. The second study asserts that a tiny fungus smaller than a human hair, known as Tortotubus, was one of the first land-dwelling organisms some 440 million years ago.
According to Dr. Martin Smith, an earth scientist at Durham University in the U.K. and the author of the fungus study, those of us living on land all these millions of years later have a lot to thank this fungi for.
"It's amazing to think that whilst the seas were teeming with complex life, the surface of the land was barren and void," Smith told me. Smith said that to survive on land organisms faced a number of unique challenges, including UV exposure from a brighter young sun without much ozone to block it, a lack of nutrients, the unfamiliar role of gravity, and temperature variation.
"The transition to land probably started with organisms that were able to survive being stranded each high tide, or being left high and dry for short periods as lake or river levels fluctuated," he said. Once happily relocated on land, Tortobus and other similar organisms paved the way for bigger life forms by enriching soil and creating an environment that more complex plants and fungi could take root in.
"By decaying the remains of whatever else was living on land—possibly algal crusts or lichen-like organisms—Tortotubus would have contributed to the earliest carbon and nitrogen cycles, liberating nutrients from dead organic matter by converting them to an accessible form, and thus 'fertilizing' the surface of the land," said Smith.
The oldest fossils from land-based creatures date from the late Ordovician, a geologic epoch around 440-445 million years ago. By the time Tortubus went extinct 70 million years later, the first trees and forests had come into existence, according to Smith, who said life on land had "transformed from simple crusty green films to a rich ecosystem that wouldn't look out of place in a tropical greenhouse today."
While life has existed on Earth for nearly four billion years, up until about 600 millions years ago this included primarily bacteria, plankton and tiny algae. Then, during what is referred to as the "Cambrian explosion," a burst of diversification happened about 570 to 530 million years ago. While the Tortubus fungus came into being well after this evolutionary bonanza, a genetic analysis by MIT recently found a molecule from a sea sponge present in 640-million-year-old rocks, making this creature one of the first animals known to exist.
“There’s a feeling that animals should be much older than the Cambrian, because a lot of animals are showing up at the same time, but fossil evidence for animals before that has been contentious,” said David Gold, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences and lead author on the study, in a statement. “So people are interested in the idea that some of these biomarkers and chemicals, molecules left behind, might help resolve these debates.”
Gold told me that animals, including sponges, and fungi separated from each other as long as a billion years ago. He said both groups evolved in the oceans before independently adapting to life on land. He said animals like the sponge had no incentive to move onto land before it was colonized by plants and fungi, because otherwise there wouldn't be anything else to eat.
Published in the Proceedings of the National Academy of Sciences, the researchers focused on a particular molecule, 24-isopropylcholestane, that modern sea sponges produce. The team looked through the genomes of about 30 different organisms, including plants, fungi, algae, and sea sponges, in reaching their conclusion.
Like many groundbreaking scientific findings, these studies give rise to at least as many questions as they answer.
“This brings up all these new questions: What did these organisms look like? What was the environment like? And why is there this big gap in the fossil record?” Gold said. “This goes to show how much we still don’t know about early animal life."
One thing we do know is that rapid changes to the makeup of the Earth's atmosphere, such as an increase in carbon dioxide and a lack of oxygen, have played large roles in past mass extinctions, including those during the tumultuous Cambrian era. With greenhouse gas emissions increasing rapidly due to the burning of fossil fuels, humankind is conducting a geologic experiment of its own in what has become known as the Anthropocene era, i.e. the age of humans. Many scientists already consider the planet to have entered into a sixth mass extinction.
Gold told me that the current trend of ocean warming is especially worrisome to him because warmer water looses gas suspended in the liquid.
"This means that modern oceans are not only at risk of emitting more carbon dioxide into the air—which exacerbates the climate change problem—but also loosing oxygen," he said. "In a low-oxygen ocean, the animals that evolved in such conditions like sponges and jellyfish will probably do well, but potentially at the expense of fish and marine mammals."