Meet Bicellum brasieri, a tiny, spherical animal measuring barely a thousandth of an inch discovered in the highlands of Scotland. Bicellum lived a billion years ago, yet remains a critical precursor for all animal life still to come, including ourselves. Fast forward a billion years, and scientists have also discovered the rules that govern the growth of everything from teeth to the thorns of a rosebush.
Somewhere around 541 million years ago, multicellular life exploded on the scene, an era now known, aptly, as the Cambrian Explosion. Over the course of a relatively short period, 13-25 million years, all of the modern phyla of animals begin to appear in the fossil record. It was as if modern life and then some suddenly sprung into existence all at once.
The fossils from the Cambrian Explosion were so remarkable and plentiful it was known before even Darwin’s theory of natural selection. William Buckland first noted the appearance of a wide range of fossilized animals as early as the 1840s. Since then, scientists have discovered a stunning variety of life during the period. The most common fossil was of an arthropod, Marrella, one of the ubiquitous trilobites. Marrella was clearly related to modern insects, arachnids, myriapods, and crustaceans, but also different. About 2 cm or less in length, the organism has a head-shield with 2 pairs of long spikes pointing to the rear. There are also two pairs of antenna mounted underneath the head, one long and sweeping, the other short and stout.
Other forms are stranger still. There is a five eyed arthropod, Opabinia, about twice the size of Marrella and a carnivore with the mouth facing backward. Opabinia also had lobes along its body and a fan-shaped tail. Wiwaxia was a smaller, soft bodied animal that was covered in scales and spikes. From above, you cannot tell the front from the back. From the rear, it was almost rectangular. On the bottom was a slug-like foot. Hidden on the front was a strange mouth with two or three rows of backward pointing teeth.
Many of these strange life forms ultimately died out, others went on to become the animals that exist to this day. It was as if evolution itself embarked on an amazing experiment, spinning out life form after life form for twenty million years before settling down into a more stately pace. While the existence of the Cambrian Explosion itself was rarely in dispute as the fossils are there preserved in stone for all to see, two puzzles remained: What caused life to move forward so fast after billions of years of much simpler, mostly single-celled organisms and what multi-celled organisms came before?
The causes are incredibly difficult to answer. More than likely, it was a wide variety of factors. It was the first period in Earth’s history where oxygen became widely available, providing primitive animals with a relatively abundant energy source. It was also the period where the ozone layer first appears, shielding more complex, multicellular animals from solar radiation. Interestingly, it’s also a period marked by an increase in calcium in seawater, creating a supply of raw materials to build bodies. There are also purely developmental factors, namely the appearance of the Hox genes and the ability for organisms to evolve their body plan.
What came before has also been incredibly difficult to answer, at least until now.
The problem here is pretty simple: Microscopic organisms without bones, shells, exoskeletons, etc. leave very few fossils. Often, we are only able to determine their existence based on their impact on the environment, looking for clues like a detective at a crime scene. Recently , however, scientists have discovered a tiny fossil in the Scottish Highlands. The fossils are of a roughly spherical organism less than 30 micrometers across, one one thousand of an inch, believed to have lived about a billion years ago, four hundred million years before the Cambrian Explosion. “We have found a primitive spherical organism made up of an arrangement of two distinct cell types, the first step towards a complex multicellular structure, something which has never been described before in the fossil record.” explains paleobiologist Charles Wellman of the University of Sheffield in the UK.
Amazingly, the stone deposits at the site of an ancient lake have kept the fossils remarkably well-preserved. The microscopic structure of the organism is visible in several specimens. The new organism has been named Bicellum brasieri. It consists of a small sphere of tightly packed cells, also spherical in shape, surrounded by a single outer layer of more sausage shaped cells. The details are so fine scientists were able to identify two populations, though they believe one form is the juvenile and the other the adult because the sausage shaped cells have not yet formed around the exterior of the central stereoblast.
Although scientists have identified fungus and algae from the same period, this appears to be the first Holozoa, a group that contains modern animals, meaning the tiny Bicellum is a precursor to both the Cambrian Explosion and ourselves. “The origins of complex multicellularity and the origin of animals are considered two of the most important events in the history of life on Earth, our discovery sheds new light on both of these,” Dr. Wellman explains.
The organization of the organism into two different types of tissue, the spherical and sausage shaped cells, and the record of one growing into the other also provides insight into the origins of complex traits in later animals. “Biologists have speculated that the origin of animals included the incorporation and repurposing of prior genes that had evolved earlier in unicellular organisms,” explains paleobotanist Paul Strother of Boston College. “What we see in Bicellum is an example of such a genetic system, involving cell-cell adhesion and cell differentiation that may have been incorporated into the animal genome half a billion years later.”
Elsewhere, another group of scientists solved a 350 year old puzzle also related to animal development. Why do so many features in nature in so many different organisms take the same general shape? From teeth to tusks, horns to beaks and shells in animals and even thorns and prickles in plants, everything seems to follow the same tapering, curved plan, thicker at the base, tapering to a point, and curving slightly or a lot as it tapers.
Why is that the case when many of these animals don’t share the same genes or are only distantly related to one another?
The answer is what is known in statistics as a power law. A power law is a relationship between two quantities where a change in one quantity causes a proportional change in the other, regardless of the initial value of the quantities. In other words, one quantity varies directly as a function (technically a power, often logarithmic) of the other. The area of a square is a simple example: If you double the length of the side, the area increases by a factor of four because the area is the length of the side squared.
Power laws themselves are everywhere, covering everything from the foraging pattern of animals to the frequency of words in many languages. It has long been known that evolution takes full advantage of them to grow a mature, multicellular organism, whether a plant or an animal. This is because DNA is more like a recipe than a blueprint; an organism isn’t made or built, it’s grown from the ground up, meaning your DNA doesn’t say put the tip of a person’s nose one inch from their face or contain a mold to grow their teeth half an inch and stop.
Instead, it takes advantage of ratios and power laws to provide stability in the developmental process. Previously, scientists had discovered how a logarithmic spiral explains some of these structures, but the spiral approach only described the overall shape, not how the shape is generated in nature.
In a new study, a research team at Monash University has demonstrated that a “power cone” is responsible for the shape of everything from teeth to thorns. The power cone uncovers a unique relationship between the radius of the structure and its length, specifically when the radial growth rate is unequal to the length. The researchers applied this power law to the teeth of giant sharks, T-rex, mammoths, and humans, and found the same pattern at play. They also looked for it in claws, hooves, horns, spider fangs, snail shells, antlers, plus the beaks of birds, dinosaurs, and mammals, and the result was the same. It even applied to thorns on a rose bush and a lemon tree.
In addition to explaining the formation of the shape, the new power law can be used to calculate the age of mammals with teeth that grow throughout their lives such as elephants and rodents.
“The diversity of animals, and even plants, that follow this rule is staggering,” Associate Professor Alistair Evans from Monash University and the leader of the research team explained. “We were quite shocked that we found it almost everywhere we looked across the kingdoms of life — in living animals and those extinct for millions of years.” The discovery is especially exciting because the idea was first proposed by the designer of London’s St. Paul’s Cathedral way back in 1659. At the time, Christopher Wren, a renaissance man whose interests included anatomy, physics, and mathematics, suggested that the shell of a snail could be made by twisting a logarithmic spiral.
“This new rule is the missing piece of a 350-year-old puzzle of how animals and their parts grow,” Professor Evans noted. “Because so many structures follow this growth pattern, we can use it to predict the likely pattern of evolution. Whenever animals evolve teeth, horns, or claws, it seems most likely that they will be this shape. It even allows us to predict what mythical animals would look like if they follow the same patterns of nature.”
From before the Cambrian Explosion to the world of today, life has taken advantage of nature’s patterns to evolve and grow, combining evolution with rules of growth that leverage complex mathematical principles to build ever more complex bodies. While the tiny Bicellum, dating back a billion years, didn’t yet use the power cone, life in the Cambrian Explosion almost certainly did, meaning as of about 500 million years ago both the genetics of the body plan and the rules for growth were in place, leading directly to the wonders of life today. That some of the genes and the rules themselves are both widely shared and incredibly ancient shouldn’t suprise us: Evolution is the ultimate miser, wasting nothing and repurposing everything, uniting all life across a few chemicals and building blocks.