Ask anyone what makes us human, and you will probably receive the answer that it is our brains – our oversized, convoluted, magnificent brains with their 100 billion neurons and 100 trillion connections – that have gifted us our unique position in the animal world. The human brain is considered by many to be the pinnacle of evolutionary design, being a highly efficient, immensely flexible and mind-bogglingly quick computing machine. Unravelling the workings of this machine is a daunting challenge that many bright minds have nevertheless accepted over the years, and we now have a basic, if incomplete, understanding of the basic principles along which the brain functions.
What exactly is the brain?
The brain is, anatomically speaking, a mass of concentrated nervous tissue. Nervous tissue consists of specialized cells called neurons, whose most significant feature is that they are electrically excitable. This means that these cells have electrical properties that can be modulated by incoming and outgoing signals. In addition, nervous tissue contains other cells and non-living structures which support the survival and function of these neurons. Acting through sets of tightly regulated electrical signals, the nervous system receives information from the outside world and sends out messages to different body parts to control and coordinate their actions.
All our sensory systems, including sight, smell, hearing, taste and touch feed their information into the brain. The brain, after carefully sorting, reconstructing, interpreting and integrating this information, decides what action to take. It then directs messages along carefully laid tracts of neural fibers (called nerves) to the part of the body that needs to act upon this new information. In their most fundamental form, the brain and the rest of the nervous system help the various cells of a multicellular organism stay in ‘touch’ with each other, and act in a concerted, coordinated manner. Be it moving away from a predator or moving towards a potential prey or a mate, it is the nervous system that processes all the information and decides on the necessary course of action.
Do all animals have brains?
Surprisingly, the answer is no. Sponges, which belong to the phylum Porifera, are among the most evolutionary ancient animals known and they lack any signs of a brain or even a nervous system. Their bodies, which consist of loosely bound cells embedded in a gel-like matrix, remain sessile (stuck to one place) for most of their lives and require little coordinated action. Reports do suggest that sponge cells sometime signal to each other through hormones or chemical ions like calcium to allow local contractions. However, even rudiments of structures similar to the brain, or any cells similar to the neurons are completely absent.
The first primitive nervous systems appear in jellyfish, comb jellies, and their relatives who fall into two broad groups – Cnidarians (jellyfish, hydra etc) and Ctenophores (comb jellies). These animals, while much more advanced than the simple sponge, also lack a brain. Instead, they have what is called a ‘nerve net‘, neurons spaced more or less evenly apart and connected to each other via long fibers. Even in these organisms, local concentrations of neurons can be found near sensory organs that show a degree of functional specialization. However, there exists no centralized point for controlling information inflow or outflow.
The ‘brain’ appears for the first time in evolution in small worms and worm-like creatures. These animals have a body plan with bilateral symmetry (which means that the left and right halves of the body are mirror images of each other), similar to 99% of all modern species of animals. In animals with bilaterally symmetrical body plans, all the nervous tissue including specialized organs for sight, hearing, smell etc, gradually started getting accumulated at one end of the organism during evolution – giving rise to the head and the brain within it in a process called cephalization.
A notable exception to this rule is the phylum Echinodermata – which consists primarily of starfish, sand dollars and sea urchins. Echinoderms appeared much later on the evolutionary time scale than jellyfish and worms, and evolved from animals with bilateral body plans. However, these animals adopted a pentaradial body symmetry instead of a bilateral one (i.e. their bodies can be divided into five equal divisions), and in the process, got rid of their brains. This is not to say that the nervous systems of starfish are simple – in fact, it is quite the contrary. Starfish have complex neural networks along each of their arms, and a specialized nerve ring in the center, alongside separate sensory and motor systems. At some point in evolution, these animals lost their brains, and seem to be surviving quite happily without it.
Cost of maintaining a brain
This leads us to a question – why would any species not want a big and specialized brain? Shouldn’t evolution have favored individuals with bigger brain sizes and allowed them to survive and reproduce better? After all, a brain allows you tremendous flexibility in learning and adaptation, and allows the formation of complex survival strategies. Why did only one small branch of primates have this massive expansion in brain size as a part of its evolutionary history?
The truth is – having a big brain is immensely costly from an energetic point of view. The brain accounts for hardly 2% of the body mass in humans, and uses up 20% of all the energy consumed. The brain functions continuously, and for that it is continuously hungry – much of the food we eat goes towards the maintenance of the brain, and the brain alone. In situations of resource limitation, restricting the brain size makes sense, and many species have stuck with this. In fact, in some birds, the brain shrinks and expands in a seasonal fashion, to allow them to cope with challenges such as learning and singing courtship songs during mating season, and to get rid of the extra brain tissue that they don’t need in the off-season.
How do our brains control our bodies?
The brain, as we have seen so far, is a concentration of nervous tissue that has the capacity to receive, integrate and disperse information to every part of the body and control the functioning of the organism as a whole. Our brain is a marvel of evolutionary complexity that allows us to think, dream and philosophize; to perceive the world through sight, smell, hearing and touch; to love, fear, hate and empathize; to breathe, move, sleep and pump blood through our bodies.
It does most of these primarily by two means – by passing on electrical signals to bundles of neural processes called nerves, which in their turn signal to muscles (again by passing electrical currents) allowing the muscles to contract or expand, leading to almost every movement-based action that we can take – voluntary ones including walking, talking, and making facial expressions, as well as involuntary ones such as breathing, blinking and the beating of our hearts. The second way is via controlling the release of small molecules called hormones, chemicals which are released by different tissues into the bloodstream and have a variety of effects upon other tissues upon reaching them.
Does the brain control everything?
It is important to realize that the brain is only a part of, and not the entirety of the nervous system. It is the most complex neural organ there is, and it receives input from virtually every part of the body. However, in many cases, the brain functions more as an override system than a system for continuous, hands-on control. Nerves originating outside the brain maintain and regulate many of the involuntary functions necessary for survival, for e.g. breathing, heartbeat etc – these form the autonomic nervous system. Specialized nerve cells also control the reflexive movement and signaling in the gut – this is called the enteric nervous system. And finally, small reflex actions, like the knee-jerk reflex when your doctor hits below your knee with a mallet, or when you quickly move your hand away from a hot object, occur in loops within the spinal cord without ever reaching the brain.
To conclude, the brain is a specialized part of the nervous system, which carries out the function of control and coordination in most animals. That being said, it is one of the most intricate and complexly designed structures that have ever existed, and deserves intense and extensive study.
Next week, we will learn in more detail about the way the human brain evolved, and the parts of it which are remarkably similar to our humble ancestors. If you have any questions, or want to share any interesting facts you know about the brain, feel free to post in the comments below.