Rising from the deep

Life under the sea is mysterious. Creatures from the deep may look like aliens from a different planet, but it is quite surprising to find that we have a lot more in common with them than we think. It all began more than 500 million years ago when individual cells swimming around in the primordial ocean decided that they would stick together and work in harmony to increase their chance of survival on an ever-changing planet. Within this first multicellular conglomerate, some cells took care of catching food, some took charge of providing protection and structure, while others decided to become the first neurons and muscles of the first animals on Earth. These earliest animals are represented today by the sponges, sea anemones, corals, and jellyfish.

After hundreds and millions of years of evolution, animals now occupy diverse habitats and have morphed into varied forms. Today, most modern animals possess a nervous system, a complex organization of cells that makes it possible to perceive the environment and to react accordingly. For example, the nervous system of the earliest humans allowed them to track and catch their food while escaping predators. The human nervous system has also developed such complexity that it has made possible higher-order functions like language and culture.

The animal nervous system is composed of specialized cells called neurons. Neurons transmit information from one cell to another and to the cells of the brain, much like the circuits in a computer conduct electrical information to the CPU. Neurons talk to each other through connections known as synapses. At the synapse, one neuron releases small chemical messengers called neurotransmitters that are received by receptor proteins on another neuron. The release of neurotransmitters from one neuron relays the signal to the next neuron, which relays the signal to the next, and so on. In order for this process to occur rapidly, the many proteins that make up a synapse have to function together like a well-oiled machine. Synaptic activity is very important because it is through these synaptic connections that we encode new learning and store our memories.

The beginnings of the nervous system can be seen in marine invertebrates called sponges. Sponges are immobile animals that live attached to the ocean floor and eat by filtering seawater for particles of food. While most modern animals possess a fully developed nervous system, sponges are an exception because they simply do not have one. Thus, when the sponge genome was sequenced, scientists were astonished to find that, despite their simple form, sponges possess within their genome almost the same set of genes as humans. In fact, sponges possess many of the same genes that make the proteins needed to build the human synapse. This was a rather puzzling discovery and it is only now that we are beginning to understand what this means for the origins of the nervous system.

Any machine is made up of many different pieces, all of which have to be present and assembled together in a precise manner. Take a car, for example. The chassis, the engine, the wheels, and the body, all have to be put together in a specific way and in proper sequence to build a functional car. We can imagine that a similar process is involved in building the synapse, except that evolution does it in a random manner, relying on adaptation and natural selection, over a very long period of time. Upon closer inspection of the sponge synaptic proteins, we realized that although a lot of the pieces were present in the sponge, some critical bits were still missing. Most notably, the proteins that keep synaptic junctions together were not there, nor were the proteins needed for receiving neurotransmitters. Furthermore, the existing proteins did not appear to have the ability to interact with each other, particularly because they were made in different cells and at different times during sponge development. This tells us that the sponge would have been unable to form a structure that could function like a synapse, even though a partial blueprint for its construction was already present.

The first operational nervous system, in the form of a nerve net, emerged in the ancestor of the cnidarians. Cnidarians, such as corals, sea anemones, and jellyfish, are the first true animals with distinct tissue types and organ systems. Within its neurons, the coral has a full set of synaptic proteins that interact with each other, thus allowing the assembly of working synapses. Evolution has given corals the complete blueprint for building a functional nervous system, including what parts to produce, when to produce them, and how to put them all together. In contrast to the passive lifestyle of sponges, the presence of a nervous system in cnidarians allowed them to have a more active lifestyle, hunting and catching other animals for food.

The impetus to evolve a more complex nervous system resulted from the demands of surviving in a changing environment. As animals and their nervous systems continued to evolve, the components of synapses and neurons were further refined. Thus, the animals that we see today have sophisticated nervous systems that are best suited to their habitats and behaviors. As for the sponge, it gets along just fine without a nervous system. Sponges have developed clever ways to adapt to the diverse environments in which they live. For example, they have evolved different shapes to deal with ocean currents or the availability of food. Also, because they are attached to marine surfaces and cannot swim away from danger, they have developed innovative chemical weaponry to protect themselves from competitors and predators. What then are the precursors of synaptic proteins doing in the sponge? This and many other questions still remain unanswered. We have more to learn about how the diversity of life arose from the deep blue sea.

Whether it is the synapse or the complexity of life on Earth, there is plenty of evidence to show that Nature is continuously innovating and adapting to an endlessly changing environment. New functions are created by mixing and matching different parts of genes, or by duplicating genes and allowing the copies to form new interactions, or by shuffling around pieces of the genome to change the way that genes are expressed. The appearance of a carefully designed machine, like a synapse, a neuron, or a brain, turns out to be just a happy accident of evolution.

* * *

Cecilia Conaco is an assistant professor at the Marine Science Institute of the University of the Philippines, Diliman. She was a graduate of the Molecular Biology and Biotechnology program at UP Diliman and spent several years as a research assistant at the UP Marine Science Institute. She obtained a Ph.D. in Molecular and Cellular Biology from Stony Brook University, New York, in 2007. As a postdoctoral research fellow at the Neuroscience Research Institute of the University of California, Santa Barbara, she used a comparative genomics approach to study the evolution of neuronal synapses in early animals and performed next-generation sequencing to examine the transcriptome profile in a marine sponge. She would like to acknowledge Vito Butardo Jr., Raymond Regalia, and Ma. Theresa Domine for insightful comments on this article. You can e-mail her at [email protected].