Your Inner Fish

In graduate school, Shubin studied how the cells of a salamander or frog come together to make bones, by staining the cells with dyes that turn bone red and cartilage blue. Shubin found that specific clumps inside the limb bud of the embryo became bones. Somehow, the cells are able to communicate and attach to one another in order to make specific materials. Shubin asks how the cells “know” how to come together to make a body at all.

Habeas Corpus: Show Me the Body. Mats of bacteria or groups of skin cells are not enough to be called a body, though they are also clumps of cells that work together. To be a body, all the cells in a clump have to work together and have a specific portion of the body that keeps the entire clump alive. In a body, some clumps of cells are specialized for different kinds of labor, such as hearts, brains, or stomachs.

Yet despite the interdependence in a body, some cells in the body can die off and be replaced in a way that keeps the entire body working seamlessly. Diseases like cancer happen when some body cells don’t know when to die, or when to stop growing. This balance means that cells had to learn how to work together. At some point in the history of life, cells developed a mechanism for doing revolutionary things like communicating, sticking together, and trading proteins.

Digging Up Bodies. For most of the Earth’s history, life was only single-celled organisms. If all of the Earth’s history were reduced to one year, single-cell organisms would be the only life until June. Animals with heads appear only in October, and humans do not develop until December 31. Fossils of the earliest organisms with bodies were actually found in the 1920s and 30s, but paleontologists did not know what they were looking at. These bodies just looked like disks and plates.

In 1947, Reginald Sprigg found many rocks with impressions of disks, ribbons, and fronds in the Australian outback. Most paleontologists gave them little thought because it was thought that the rocks came from the young Cambrian era when animals already had bodies. In the 1960s, however, Martin Glaessner proved that these rocks were actually 15-20 million years older than originally thought. These rocks actually held the impressions of the some of the earliest bodies ever formed.

Sprigg’s rocks show that multi-celled organisms with some sort of bodily symmetry and body system specialization had appeared by 600 million years ago. The rocks also show trace pathways of movement, showing that these early bodied creatures were able to move in ways different from the movement of bacterial mats. Now that Sprigg’s rocks show when the first bodies developed, Shubin turns to how and why bodies would happen.

Our Own Body of Evidence. Though humans may seem to have nothing in common with the early Precambrian bodies, those early bodies were actually made out of the very material that allows human body cells to stick together. In the human body, this biological “glue” is a complicated mix of molecules that differs depending on the organ that it is holding together (e.g. an eye or a muscle). Without these molecules, bodies would not even be possible.

Shubin now dives into how bone tissue is connected, as bones are essential to keeping the human body moving and functional. Bones are like a bridge made of steel or cables—only as strong as its building blocks. Yet bones also have to be slightly bendable so that the human body can move and withstand force. The specific balance of strength and flexibility in the human skeleton is what allows humans to run, just as a frog’s skeleton is specialized for jumping.

Looking at the bones under a microscope reveals the molecular structure that gives bones their strength. Some cells are tightly packed together while others are separated. Where cells are separated, minerals such as hydroxyapatite help give bones strength when they are compressed. In the gaps, a ropelike bundle of fibers called collagen gives bones strength when the collagen is pulled.

Cartilage behaves differently than bone, as a much softer material that bends when force is applied and then springs back to its original shape (when healthy). At a molecular level, cartilage has much more space between its cells, with lots of collagen filling in and an incredibly specialized molecule called a proteoglycan that can fill up with water to cushion the cartilage cells to withstand force. Like bone, the material between the actual cartilage cells gives the cartilage most of its distinctive properties. Even when different body systems have the same materials in between the cells, the ratios of different materials can change how the cells behave.

Moving back to the earliest bodies, almost all animals with a body seem to have collagen and proteoglycans in between their body cells. The earliest creatures with bodies would have had to make these materials in some way. Furthermore, the earliest bodies would have had to find a way to stick cells together and communicate in between cells.

Starting with how cells stick together, there are many different methods for connecting cells. Bone cells attach like a rivet with a molecule that binds to the outside of two cells. Some of these molecular rivets are able to selectively bond only to certain cells, helping to organize which kinds of cells belong in specific places in the body and keep cells of the same type close to each other.

Cellular communication is another important issue, as cells have to know when to divide or die in order to keep the whole body healthy. Cells send molecules back and forth to each other that transmit certain messages. A molecule will attach to the outside of a cell, setting off a chain reaction of molecules in the cell until the message reaches the cell’s nucleus. Shubin hopes to find the first bodies where these mechanisms of cell attachment and communication were in place.

Bodybuilding for Blobs. In the 1880s, workers at an aquarium found a living mass of goo on the glass walls of the fish tanks. This blob is now known as a placazoan, a very simple creature with only four different types of cells in its plate-shaped body. Yet though placazoans are simple, they do have the necessary features of a real body – namely division of labor among the different parts of the body. Placazoans also have rivet connection and cell communication tools between cells.

Going even further back, sponges have bodies that are simpler than placazoans. The “body” of a sponge is actually a non-living silica complex with collagen interspersed. In 1894, H.V.P. Wilson, the first professor of biology at the University of North Carolina, found that sponges could even properly put themselves back together if their bodies were dispersed through a sieve.

It is the cells within the sponge that make sponges truly interesting. Special cells shaped like goblets direct water through the sponge while tiny “arms” branching off from these cells catch food particles for the sponge. The goblet cells also have flagellum (like tiny cellular legs) that can beat in tandem to move a current of water through the sponge. From this, we can see that sponges have a very primitive version of the organization of labor in the human body.

Placazoans and sponges are as simple as bodies can get. To find out anything more, Shubin must turn to single-celled microbes. For years, scientists assumed that the genetic information of microbes would be completely different from animals with bodies, as these cells have none of the adhesion or communication abilities that body cells have. Yet Nicole King changed that by studying choanoflagellates, the closest microbe relatives of placazoans and sponges.

Choanoflagellates look like tiny versions of the goblet cells in sponges, but their DNA is actually more similar to microbes. Choanoflagellates then form a link between single-celled microbes and organisms with bodies like sponges. Choanoflagellates also have the molecules that could be used for cell adhesion or cell communication. Expanding her research on microbes, King then found primitive versions of collagen and proteoglycan on the surfaces of different microbes that specialize in invading and infecting other cells.

A Perfect Storm in the Origin of Bodies. With King’s research, it seems that the building blocks for bodies were in place long before bodies actually appeared. The actual timing depends on many factors. One theory for the development of bodies is that microbes banded together to avoid being eaten by bigger microbes. Molecules that microbes use to catch prey could potentially turn into the molecule that stick cells together in the body.

Researchers did an experiment to support the predator explanation for body formation. After cultivating a single-celled alga for thousands of generations, biologists introduced a predator that caught and ate single-celled microbes. The alga clumped together, finally stabilizing into groups of eight cells that were big enough not to get caught but small enough that each cell could still get enough light to survive.

If predators are a viable explanation for the emergence of bodies, we must look to other factors to explain why bodies took so long to develop. The ancient environment was much harsher than our current environment, and bodies are hard to maintain. Collagen especially requires a lot of oxygen, meaning that cells would have needed a huge surplus of oxygen to even consider producing that molecule. A billion years ago, Earth’s oxygen levels spiked, possibly giving microbes the extra resources they needed to begin forming the building blocks for bodies.

Shubin now has the “when”: 600 million years ago, the “how”: adhering together through molecular rivets and communicating with molecular messengers, and the “why”: bodies are big enough to allow microbes to avoid predators. When the environment reached high enough oxygen levels for microbes to put all of these tools into practice, bodies developed and life on Earth changed forever.