Virtually every definition of the word “life” includes the capability to undergo Darwinian evolution as a necessary characteristic. This is true of life on Earth and of thinking about what would constitute life beyond Earth. If it can’t change, the thinking goes, then it cannot be truly alive.
In addition, evolutionary selection and change occurs within the context of broad planetary systems — the chemical makeup of the atmosphere, the climactic conditions, the geochemistry and more. If an environment is changing, then the lifeforms that can best adapt to the new conditions are the ones that will survive and prosper.
So evolution is very much part of the landscape that Many Worlds explores — the search for life beyond Earth and effort to understand how life emerged on Earth. Evolution happens in the context of broad conditions on Earth (and perhaps elsewhere), and finding potential life elsewhere involves understanding the conditions on distant planets and determining if they are compatible with life.
This all came to mind as I read about the discovery of a remarkable collection of fossils alongside a river in China, fossils of soft-bodied creatures that lived a half billion years ago in the later phase of what is termed the the Cambrian explosion. They are of being compared already with the iconic “Burgess Shale” fossil find in Canada of decades ago, and may well shed equally revelatory light on a crucial time in the evolution of life on Earth.
The new discovery is reported in the journal Science in a paper authored by Dongjing Fu and a team largely from the Northwest University in Xi’an. The paper reports on a zoo of Cambrian-era creatures, with more than half of them never identified before in the rock record.
The animals are soft-bodied — making it all the more remarkable that they were preserved — and some bear little resemblance to anything that followed. Like the Burgess Shale fossils, the Qingjiang discovery is of an entire ecosystem that largely disappeared as more fit (and predatory) animals emerged.
The Qingjiang ecosytem, which is inferred to have existed in coastal waters, includes corals and jellyfish, sponges and arthropods such as insects and crustaceans, and small fish-like creatures (chordates) that are the earliest members of our own phylum.
The fossils have been dated to about 518 million years ago, making them about 10 millions years older than the Burgess Shale fossils. This period is when life on Earth was transformed in a relatively short time from mostly simple and small to far more complex and larger. Some call it the most important period in Earth’s evolutionary history.
Most animals before this evolutionary “explosion” were stationary and lived in microbial mats. A few meandered blindly over the slime, grazing on the microbes. Life at this point was largely single-cell, simple, and there were no predators.
But within a few million years, life on Earth became far more complex with highly mobile animals that featured modern anatomical features and lived in diversified ecosystems. The Cambrian explosion produced arthropods with legs and compound eyes, worms with feathery gills and swift predators that could crush prey in tooth-rimmed jaws.
The precise cause of this explosion has remained elusive, in part because so little is known about the physical and chemical environment at that time. But there are some points of consensus.
The increasing amount of oxygen in the water and atmosphere — produced as a byproduct of floating cyanobacteria — is generally agreed to have played an important role. The concentration is theorized to have become great enough that more complex animals could develop with the energy the increased oxygen provided. But other potentially contributing factors include the ending of a “snowball Earth” event during which the planet was largely ice and snow covered for millions of years, and the development of some key evolutionary innovation, such as vision (which in terms allowed for greater predation.)
As described n the Science paper, the site offers a remarkable diversity of early life that is very well preserved. Many of the creatures were mobile or had developed shell-like coverings to protect themselves. The site is especially rich with Cnidaria — jellyfish, coral and sea anemones — all with specialized cells used primarily to capture prey.
Qingjiang also provides a look at how environmental factors dictate what life survives and why. The site is about 600 miles away from another Cambrian Explosion fossil site called Chengjiang, which has fossils from almost the same time period and from the same geological formation. But the overlap in animal types between the two is small — only about 8 percent of species are found in both. What appears to be different, the authors say, is simply that the Qingjiang animals lived in deeper water.
These similarities and differences of the two Chinese sites are among the key features of the site.
“There are always differences in the exact species found at each Cambrian Explosion fossil site, but it is usually hard to pin down why they are different,” co-author Xingliang Zhang said in an email. ” When compared to the Chengjiang, we remove a lot of factors that could be responsible for faunal differences – they are the same age, and they are found on the same paleocontinent, so these factors are probably not explaining why the taxa found at each site are so different.”
“The major difference between the sites is related to the environment, with Qingjiang being in a deeper marine setting than the Chengjiang. So the researchers on this site will be able to see how water depth affects the type of taxa that live together.”
In an accompanying Science commentary, Allison C. Daley, a Cambrian era paleontologist University of Lausanne, wrote that Fu et al “convincingly demonstrate that the Qingjiang biota represents an assemblage of organisms that was preserved nearly in place, providing a snapshot of a real animal community 518 million years ago. ”
In an email, she said further that the newly revealed site “tells us about the level of biodiversity present in the early Cambrian. As one of the oldest sites, the Qingjiang gives us one of the earliest peaks at animal diversity, and we see that these ecosystems are already very complex with a diverse set of creatures living together, and with complex ecological interactions in place ”
She was especially struck by the presence of so many jellyfish (Cnidaria) and even simpler creatures called comb jellies (Ctenephora.) These very early invertebrates share the “jelly” and water composition of jellyfish, but are considered more primitive and often don’t have tentacles.
Comb jellies are often viewed along with sponges as the first creatures that could be called “animals.” Daley said that Qingjiang offers one of the best opportunities so far to dig deeper into the question of which might have been the first animals.
Given the fact that many of the fossils are of creatures without shells or bones, how were they preserved for so long? Or at all?
This question of preservation — the field of taphonomy — looms large in both paleontology and planetary science and planetary habitability. Researchers working on Mars life-detection missions, for instance, are now spending a great deal of time learning about what conditions on Mars might be most likely to preserve remnants or signs of potential earlier life.
In the case of Qingjiang (as well as Burgess and Chengjiang) are “Lagerstatten” — fossil sites exhibiting extraordinary preservation and often animal or plant diversity.
The mechanism for preservation is generally the burial of the lifeforms in an oxygen-free environment with minimal bacteria, thus delaying decomposition. This is theorized to have happened at these Cambrian sites when an upslope mudslide carried the creatures to lower depths and then entombed them in the mud.
Daley was especially impressed by the quality of preservation at Qingjiang. “These fossils having some of the most detailed and exquisite preservation of any Cambrian Explosion locality. They are all characterized by having soft parts preserved, but at the Qingjiang the preservation is almost the best known.
As Xingliang Zhang tells it, the Qingliang discovery was the result of both good fortune and being at the right place.
He and his students were looking for the kind of Cambrian-era rocks that could preserve soft tissue in central China. They were exploring a certain formation without success. But on a very hot day they went down to the nearby xxx river and sat in and along it to keep cool.
Zhang said that he looked down as he lunched and saw rocks that were precisely the kind they were looking for. They split one open and immediately found a Cambrian specimen. A student found another promising rock, split it and found another specimen. As the paperreports, the number of Cambrian-era specimen found at Qingjiang has risen to 4,351.
And that is just the start. Researchers from around the world will be studying the creatures dug out from the Qingjiang site to learn more about how life took such an enormous evolutionary leap a half-billion years ago. The lessons will be many;