Exploring the Possibilities of Life Without a Brain
Sloane Boukobza
Illustrations by Michelle Shaffer
A (Cambrian) Explosion Created Our Sensory System
Around 541 million years ago, Earth’s terrestrial surface appeared barren [1]. During this time, called the Cambrian Period, large plants and vegetation had not yet evolved to cover the Earth, and life was small, simple, and squishy. Beneath the ocean’s surface, however, existed a rich and flourishing ecosystem, one which would rapidly evolve in a short period of evolutionary time thanks to one specific creature. This creature, our evolutionary ancestor, was a single-celled, microscopic organism that existed roughly 600 million years ago called the Urchoanozoan [2]. The Urchoanozoan played a foundational role in evolution by allowing for the transition from unicellular microbes to multicellular animals, paving the way for the emergence of complex human sensory systems [3]. The 10 million years following the dawn of multicellular life are referred to as the ‘Cambrian Explosion,’ a period of exponential evolution and diversification among all organisms. This accelerated diversification resulted in the many forms of life we see today, including most animals. Despite branching off from each other early on and undergoing separate evolutionary processes, many different types of animals, including humans, independently evolved to share similar traits. Some of these mechanisms, such as the neural structures that are involved in sensory processing, are even more complex in non-human animals than their counterparts in humans. The diversity within animal sensory systems presents alternate pathways and structures which facilitate everyday functions like movement and decision-making. Humans are able to learn from these systems and apply them to technology, expanding the horizons of advancement.
Neural systems have evolved independently in multiple organisms throughout history, which can be seen when comparing at which point in time different animals evolved brains. A human and an octopus both have a brain, yet the emergence of their respective brains is separated on the evolutionary tree by 550 million years, meaning they had no influence on each other’s evolution. This independent evolution of similar neural systems is considered an example of convergent evolution—when two dissimilar species evolve analogous traits to adapt to analogous environmental circumstances. Examining the complexity of neural systems, as well as considering the brain across an evolutionary timeline, serves as an excellent case study in convergent evolution. The brain is a central organ which interprets sensory information, controls body movement, and mediates behavior [4]. Insects, birds, primates, and many marine species all possess complex brain structures despite having various levels of cellular complexity [5]. None of the aforementioned animals share a recent common ancestor with a brain, yet today they all possess this structure that effectively functions as a nervous system control center. Although humans are typically thought of as having the most developed nervous system, many other evolutionarily divergent animals also feature advanced systems; these systems appear to parallel or in some ways surpass those found in humans [6]. By studying the progression of brain structures and nervous systems across animal lineages, scientists are able to uncover the mysteries of life that came before us while simultaneously pushing the boundaries of neuroscience.
Brainless to Boundless: The Spectrum of Neural Systems
Around 750 to 600 million years ago, the group Cnidaria—including jellyfish, sea anemones, and coral—split apart from the group Bilateria, a classification which includes most other present-day animals [7]. After the split, Cnidaria and Bilateria evolved independently, with Bilateria going on to encompass a wide range of animals, from humans to octopuses [7]. Although cnidarians don’t have a brain, they are widely regarded as the first organisms to have evolved a nervous system, or a system of cells that relays signals throughout the body [8, 4]. The nervous system found in cnidarians is known as a diffuse nervous system, which consists of a net of neurons interconnected throughout the animal’s body. It is this system that allows cells to directly communicate with each other; the nerves receive input from the senses and process motor output in the area where the nerve is located [9]. Many species within Bilateria, including humans, have instead developed a central nervous system (CNS), which processes information differently from the diffuse system. In the CNS, neurons relay information back to one central point within the body where all of the information from individual neurons is then integrated; this point is known as the brain. Although Cnidaria and Bilateria split from each other eons ago, their two respective nervous systems evolved in parallel to be able to process and transmit important information. Cnidarians lack the central brain that bilaterians have, yet their nervous systems contain similar components. Because they lack a brain, cnidarians were once underutilized in research on the evolution of complex neural networks of animals over time. Ultimately, however, their networking system has been extremely useful in better understanding the systems of more recently evolved species like humans. Cnidarians possess specific signaling chemicals which relay information throughout their body’s nervous system. Precisely identical signaling chemicals can be found in bilaterian brains, which is incredible proof of convergent evolution. Considering this overlap, the study of cnidarians is crucial in understanding the initial emergence of the nervous system as well as how the nervous system has adapted across different environments and species [10].
Adaptation is neither static nor linear. After Bilateria split from Cnidaria, many within the group developed a centralized nervous system. This structure is thought to have evolved on five to seven other occasions throughout history, including in chordates (like humans), arthropods (like spiders), and molluscs (like octopuses) [11]. On a smaller scale, within the group of molluscs, there are at least three to four cases where independent centralization of the nervous system has been thought to re-evolve. Each independent evolution of a CNS has allowed the respective species to better adapt to its environment and develop capabilities such as memory and learning [5]. Somehow, these independent cases of evolution converged to yield similar structural results within the nervous system, and understanding this from an evolutionary perspective will lend insight into the origin of the human nervous system.
The centralized nervous system of many Bilateria and the diffuse nervous system of many Cnidaria are radically different, yet there are animals which fall somewhere between the two. Cephalopods, like octopuses and the spiral-shelled nautilus, are a part of Bilateria and have a nervous system which features similarities to the structure of the human brain, including the spinal cord, hippocampus, and wrinkly cerebral cortex [12, 7]. Due to its lack of brain, the nautilus represents an intermediate level of centralization, as its system is neither fully diffuse nor centralized. Its neurons are concentrated in major nerve cords: tube-like structures that run throughout its body to relay information. However, these neurons do not report back to a brain as one does not exist [11]. In comparison to other cephalopods, the nautilus’ nervous system seems to be relatively simple—still, it offers insight into the varying brain systems that evolved in one animal group within the same environment.
Blobs’ and Brittle Stars’ Brainless Behavior
The phrase ‘structure equals function’ is used throughout scientific literature to emphasize the way in which an animal’s physiological structure serves to optimize its function [13]. In the case of analyzing the structure of the brain and the nervous system, it is the complexity of these structures which accounts for its range of functions [14]. The brain, for example, acts as a highly complex system for processing sensory information and facilitating movement [4]. In humans, its function is supported by the structure of one hundred billion neurons, along with around one hundred billion support cells [15]. A variety of non-human organisms can accomplish similar functions to the human brain while lacking this centralized structure, demonstrating alternate interpretations of the phrase ‘structure equals function.’ The brain allows for complex communication and processing throughout the entire nervous system, but what about the organisms in which a brain is not necessary?
A single-celled, neon yellow blob known as slime mold is a great example of a non-animal organism capable of complex information processing. Sharing characteristics with animals, plants, and fungi, this organism is made up of a single cell composed entirely of a network of interconnected tubes [16]. The complexity of this network of tubes within its one-celled ‘body’ has allowed the slime mold to engage in similar memory processing seen within the human hippocampus, a key brain area involved in short-term and long-term memory. By changing the diameter of each tube within its body through contracting and relaxing, the slime mold can ‘remember’ specified locations by analyzing the changing size of its tubes. The slime mold’s capacity for memory lends insight into an alternative method of memory formation [16].
The brittle star is another organism that contains no brain but is able to perform complex decision-making and movement with the help of a decentralized nervous system. Brittle stars are animals similar to starfish with five twisty, muscular legs for movement, which all communicate by connecting to a central point at the animal’s center. In animals, movement requires proper transmission and processing of sensory information, followed by nerve responses throughout the body which coordinate the body’s moving parts [17]. For example, humans require a centralized nervous system and brain to coordinate complex motor and sensory information [4]. Instead of relaying this information back to the brain, animals like the brittle star with no centralized nervous system divide and process information in order of importance [17]. For example, signals of injury to one of the brittle star’s arms will get relayed first throughout the organism, while signals of harmless plankton brushing past one of the animal’s arms would take a back seat [7]. The brittle star has evolved to have a nerve ring at its center with a nerve net running through each of its arms [7]. Like highways, each nerve net sends sensory information to the nerve ring at the center. This central ring acts as a roundabout by receiving and redirecting this information, which will be redistributed to highways running through the other arms of the star. This system allows each arm on the brittle star to function as a miniature brain; information essential for survival is sent directly to the nerve ring, while non-essential information is transmitted via the less direct nerve net throughout its body [18]. We see this hierarchical nervous system spring to action when one of the brittle star’s arms has been severed. The animal seemingly allocates the majority of the responsibility for motion to the arms farthest away from the missing appendage, minimizing the restriction of its movement.
Octopuses take it one step further than the brittle star. They demonstrate individualized control of each of their arms, but even when an arm is severed and falls loose, the detached arm can retain its regular behavior [19]. The localized nervous system within the severed arm allows it to continue grasping with its suckers for up to three hours after it detaches [19]. Observing this kind of variation in brain and nervous system functions across animal lineages has allowed us to understand these evolutionarily diverse structures. This creates opportunities for further applications within the human world, where we are limited by the same structure that has allowed us to progress this far: the brain.
Neurogenesis - A (R)evolutionary History
In the human quest to overcome aging and disease, a roadblock repeatedly presents itself: humans generally do not have the ability to regenerate neurons. The ability to create or regenerate neurons is called neurogenesis, and although this quality is rarely exhibited beyond an organism’s initial development, understanding how it works can directly translate to treating human neurodegenerative diseases such as Alzheimer’s disease [20] When the brain forms in a human fetus, a set number of neuronal cells is generated through a process referred to as embryonic neurogenesis. Only in rare contexts are additional neurons created throughout a human’s lifetime [21]. This means that once a neuron dies, the brain rarely, if ever, produces a new one to replace it. This limitation, however, is not universal throughout the animal world.
Some animals have the ability to generate neurons outside of the initial period of development, a process known as regenerative neurogenesis [22]. The discovery of regenerative neurogenesis played a revolutionary role in our understanding of evolution because neurogenesis was originally thought to halt after birth in all organisms. The starlet sea anemone stands out as a cnidarian known for regenerative neurogenesis [23]. Although this animal cannot perform neurogenesis at all points in its life, this phenomenon occurs as the anemone undergoes a physical transformation from its larval stage to its adult stage, drastically transforming the size and abilities of its body [24]. During this period of transition, the starlet sea anemone is able to direct certain cells to develop into neurons. By modifying its neuron count and composition, the anemone can adapt its nervous system to its new adult life in an aquatic environment [24, 25].
Another creature capable of regenerative neurogenesis is the tobacco hornworm, which modifies its neuron count when undergoing metamorphosis [26]. Similarly to the starlet sea anemone, the hornworm takes on a completely new physical form; it goes further, however, by restructuring its entire CNS. During the hornworm’s transformation into a moth, it utilizes unspecified neuronal cells found in an area of the brain similar to that in humans. The hornworm initiates cell death in obsolete cells in order to make room for these dividing cells, which go on to become functional neurons that will be incorporated throughout the moth’s CNS [26]. Understanding this process has allowed scientists to compare different types of neurogenesis to apply regenerative principles to humans.
Analyzing the growth of new neurons in species with such vastly different evolutionary paths evokes questions about how this information may benefit humans. The possibility of human regenerative neurogenesis remains alluring because humans are already limited in their capacity for neurogenesis. Specifically, this limited ability is seen within the dentate gyrus of the adult brain, an area of the hippocampus that deals with memory formation [27]. Division of new cells in this area leads to structures that resemble neurons in both form and function, but it is unclear whether these ‘proto-neurons’ are functional [27]. Even though the functionality of these cells remains unknown, their presence within the dentate gyrus allows for improved learning and memory, as well as enhanced neuronal adaptivity over one’s lifetime [28]. This very minimal process of adult hippocampal neurogenesis can be applied to neurological diseases where damaged neurons are unable to be replaced. Species that are already capable of neurogenesis developed this trait as a result of the immense genetic diversity that arose during the Cambrian explosion, and understanding these traits—whether in humans or tobacco hornworms—contributes to the long-term progression of research.
Conclusion
What if we, as humans, started modeling our research and technology after the organisms which evolved convergently alongside us instead of solely focusing on the structure and function of our own neural system? Every insect, squirrel, bird, and human can be traced back to the Urchoanozoan, a microscopic unicellular organism which lived before the Cambrian period. Within this timespan, animals evolved structures which suited their individual needs, yet an overarching pattern appeared. From nerve nets to cords to rings to a brain, independently evolved neural structures demonstrate repeated evidence of convergent evolution.
It is the differences between these neural systems that ignite scientific innovation. Brittle stars have the ability to redistribute roles across their arms when one is injured based on a hierarchy of priority, which is incredible considering their lack of a centralized nervous system. Octopuses have tentacles which respond in the exact same way, whether their arms are severed or attached to their body. These qualities have sparked new technology that is capable of working in harsh environments. Robots that investigate natural disaster sites or explore outer space are now able to endure severe structural damage and continue collecting data by imitating the brittle star’s adaptability [18]. The study of animal sensory systems formally established the field of ‘biomimicry,’ inspiring the invention of adaptive prosthetics controlled by the mind and nervous system [29, 30]. Although animals with brains dominate today’s planet, observing how nervous systems in animals without a brain delegate responsibility allows these organisms to serve as a blueprint for practical, innovative advances in human technology to further our society.
In addition to having drastic structural differences, certain animals are also capable of completing processes which humans cannot, such as regenerative neurogenesis, which can break scientific boundaries if fully understood. In people with Alzheimer’s disease, the minimal neurogenesis possible in the dentate gyrus is halted, exacerbating the already widespread death of neurons as the disease progresses. Extensive cell death and a lack of neurogenesis contribute to the deficit in learning and memory characteristic of Alzheimer’s [31]. These cognitive and physical deficits are associated with many neurological diseases, including Alzheimer’s disease, and are currently considered incurable. Analyzing the starlet sea anemone and tobacco hornworm’s regenerative neurogenesis allows for an advanced understanding of how an animal is able to repeatedly produce new neurons throughout its lifetime. Using this knowledge, scientists can induce neuron growth in targeted areas as an ideal therapeutic approach to counteract neurological deterioration [32, 33]. Through studying animals with these diversely evolved neural systems, humans become one step closer to understanding the world’s evolutionary past and reaching a hopeful future built upon the natural innovation of the species around us.
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