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01. Inception of Nerve Cells
The inception of nerve cells dates back to prehistoric times. Single-celled organisms called protists, including ancestors of today’s choanoflagellates, are thought to have given rise to the first multicellular animals. Cellular differentiation in these creatures allowed for the establishment of a primitive nervous system. These lung-shaped aquatic creatures possessed specialized cells that transmitted signals – the earliest forms of nerve cells or neurons. These cells, fundamental for communication within the organism, marked an innovative leap in evolution.
02. Birth of a Simplified Nervous System
Jumping forward on the evolutionary timeline, we reach approximately 700 million years ago, during the Ediacaran Period. Here, simple creatures like jellyfish and anemones appeared. These organisms possessed a rudimentary nervous system known as nerve nets, comprising basic nerve cells interlinked across their body. Although quite simple, this was a marked evolutionary leap allowing for improved response to environmental stimuli.
03. Emergence of Bilateral Symmetry
With the advent of the Cambrian explosion, about 540 million years ago, bilateral symmetry in animals started appearing. Their nervous systems also mirrored this symmetry, leading to the development of a central nerve cord, considered the precursor of the more sophisticated central nervous system in higher organisms. Certain worms and other invertebrates show such anatomical structure, leading to more organized and complex behavior.
04. Evolution of Ganglia and Primitive Brain
Fast forward to around 520 million years ago, when the ancestors of arthropods developed ganglia—groups of nerve cells. These ganglia often concentrated in one particular section of the body, forming a primitive ‘brain.’ This enabled the creatures to exhibit complex behaviors and movements, signifying a critical advancement in the evolution of nerve cells.
05. Appearance of the Vertebrate Brain
Around 500 million years ago, intense changes were happening in the Earth’s aquatic habitats which stimulated the evolution of the fish-like early vertebrates. These organisms developed a more complex brain, comprised of specialized regions responsible for processing different types of information. It was from these primitive brains that all other vertebrate brains, including that of humans, evolved.
06. Emergence of Myelination
A landmark event in vertebrate evolution was the arrival of myelination, where nerve fibres became wrapped in a fatty sheath of myelin. This occurred over 400 million years ago and was essential for speeding up the communication between nerve cells, which significantly enhanced cognitive functions and reactions.
07. The Mammalian Brain
Around 200 million years ago, we see the rise of the first mammals – small, nocturnal creatures with larger brains relative to their body size compared to their reptilian ancestors. Particular enhancements include the neocortex, responsible for high-level cognitive tasks.
08. Primate Brain Evolution
Fast forward to 60-70 million years ago, and we encounter the first primates, our closest animal relatives. Their brains show a significant increase in size and complexity, particularly in areas related to dexterity and vision. Their nerve cells, extensively connected, showcased complex cognitive functions.
09. Human Brain Evolution
The emergence of early human ancestors (Homo species) around 2 million years ago marked a significant milestone. Our hominid ancestors showcased an unprecedented increase in brain size accompanied by notable evolution of nerve cells.
10. Modern Day Nerve Cells
Today, nerve cells, or neurons, in the human brain number approximately 86 billion. Their properties are incredibly diverse, with different types of neurons responsible for different tasks. Our understanding of their diversity, complexity, and exact functions, however, is still incomplete, revealing the nerve cell’s evolution as one of nature’s most intriguing puzzles. We focus on not only their past evolution but also how these might change in response to our ever-changing environments, leading to better treatments and understanding of various neurological conditions.