How has the mammalian brain evolved since the beginning of time?

The mammalian brain has remained relatively similar structure wise over the last few hundred years, but is has seen a few large, evolutionary changes. It has a “jelly like” structure and can weigh up to 3 lbs. There are three main parts of the brain: the cerebrum (fills up most of the skull and is involved in thinking and memory), the cerebellum (co-ordination and balance control centre at the back of the brain) and the brain stem (connects brain to the spinal cord). The brain uses up to 50% of the oxygen and fuel consumed when the brain is busy, and it contains the richest and largest network of blood vessels.

Firstly, let’s explore the functions of the different structures of the brain. Within the vast network of blood vessels, the arteries, which are connected to the heart and lungs, carries around 25% of the blood in your oxygen to billions of cells in the brain. The network also contains veins and capillaries. The cortex is the wrinkled section of the cerebrum and each section is used for different functions, such as interpreting sensations, sights, sounds, smells, problem solving, thinking, planning, forming and storing memories and also voluntary movement.The brain is split in halves and the left hand side controls movement of the right side of the body, whereas the right controls the body’s left side. Overall, the adult brain contains over 100 billion nerve cells, which have branches that connect nerve cells in more than 100 trillion points, which scientists call “the neuron forest”. Signals in the form of electricity travel to form memories, thoughts and feelings. For cell signalling, the neurotransmitters can travel across synapses as “electrical charges”. The neurotransmitters are considered as “numbers”- so memories, thoughts and feelings are almost seen as “code”. This is what allows us to read words, comprehend them and say them- which can be seen through positron emission tomography (PET) which can analyse brain activity. The brain is part of the central nervous system, or the CNS, which is connected to the spinal cord down the vertebrae. The spinal cord is a cylindrical structure, located between the spinal canal and extended up to roughly 44cm, which varies based on gender. It divides into 31 different segments, including 8 cervical, 12 thoracic, 5 sacral (supporting weight of the body), 5 lumbar (spine of vertebrae) and 1 coccygeal (last bone at the bottom of the spine); this is also known as the “tail bone”. The spinal cord expands down 20 bony vertebrae and terminates at L1-L2 and L3 in children. It is anchored to the coccyx (tail bone)  by filium terminal. The other major parts include the hindbrain, midbrain and forebrain and contains the distinct “cytoarchitecture”. This is what separates the mammalian brain from other organism’s brain structure.Compared to other amniotes, the mammalian brain has an enlarged nervous system and this especially involves the forebrain. 

A research study carried out by UCL, ARU and University of Salford analysed over 1,000+ extinct and living mammal species to learn the structure of their brain. They used a timeline of 150 million years to observe how the brain size affected the large incfrease in body size. They compared ancestral homonins, the human line, and the brain of great apes. Overall, the main changes in the mammalian brain occurred after the two cataclysmic events happened on earth. After the mass extinction 66 million years ago, the researchers observed a great shift in the physical body size of the animals. These changes occurred in the lineages of bats, carnivorians and rodents alongside bats. These animals filed the empty niches from the extinct dinosaurs. Around 30 million years later, the Late Palogene period led to changes within the structures of bears, whales and primates which are undergoing evolutionary shifts and this meant that there was a profound change in brain proportion and body size. This is due to the fact that they had to adapt and change to get used to the new climate. Furthermore, the large brain size is related to increased intelligence, the enlarged brain size allows them to be more cognitive and make improved decisions, However, brain and body size are two extremely differing complexes, so according to some scientists, they cannot be generalised into a fixed statement. 

Variation is also seen as a key factor which is the driver of the shifts un relative brain sizes, which open up new opportunities about thinking and brain complex. This was discovered after executing research on a group of monkeys which share a common ancestor from Africa and South America; however, one could easily assume that these shifts may have been a response to ecological differences and pressures from the environment which lead to their changes in South America. In the parallel mammalian clades, the evolution of body and brains differed in much more disgtinctive levels. They may hsre the same migratory histories but still went through evolution in varying ways. This is further proven by research carried out by J.F Eisenberg and D.E WIlson. Time-calibrated phylogeny of mammals found that animals corresponded to 30 significantly different allometric grades. There were large shifts in slopes which proved crucial in later diversifications, especially in anthropoid primates; their stem and crown retained more until the PAleogene-Neogene boundary, which occured approximately 23 million years ago. This clearly proved different evolutionary trajectories in large-brained mammals. This was proven by studying five main mammalian groups (elephants, apes, hominists, whales and dolphins). Elephants proved to have the simplest ration of increasing brain size with increasing body size, but in whales and dolphins, this was much different. In toothed whales, it was interpreted that brain size decreased with increasing body size. In hominists and dolphins , brain size increased with decreasing body size and peak relative brain size parallels the patterns in birds. However, in smaller brained mammals, they have a much higher slope, In afrosocridis, hystricomorph rodents and marsupials have the least change in evolutionary trajectories. This shows that body-size becomes more restrained with smaller bodied animals, meaning they would evolve more disproportionately. Allometric shifts however, do not represent shifts in cognition. Although relative brain size is traditionally associated with cognitive capacities and behavioural flexibility, this is only seen as a fundamental assumption. Numerous studies were done to prove that brain size is related to body-allometry, however the results obtained proved them largely false. There are many factors which can be addressed that may not directly affect brain size or cognition,but plays an an important role. Some selective pressures that play a key role in species diversification are more directly tied to body size than brain size. Many results obtained by a variety of researchers found that the brain-body relationships has more than just selection involved in brain size. 

From an evolutionary perspective, brain sizes are quite difficult to pin point. Humans are known for having larger brain sizes; on average, primates’ brain sizes are nearly double what is expected for mammals of the same body size and over 7 million years, the size has nearly tripled. Most of this growth has occurred in the last 2 million years. To analyse ancient brains, researchers use the size and inside of anicnet skulls alongside a few rare fossils which have been preserved. Both of these give us information about their brain volume and relative sizes of the major cerebral areas, which set mammals apart from other organisms. In the first ⅔ of the homo sapien’s history, the size of their brains were relatively similar to the size of the ape brain today; the famous “lucy fossil” had internal brain volumes of 400-500ml, whereas the human brain had 500-700ml. By comparing this structure, we were able to observe the neocortex which had began to expand and the regions of the brain began to expand much more than just that of a visual aspect. In the final ⅓ of our history, we saw a much larger increase in human brain size. The brain expanded to connect the language processing frontal lobe ofbthe Broca’s area and the brain was averaging a bit larger than 600ml volume. Since 500,000 years ago, the volume increased 1,000 ml and today, the range averaged 1,200 ml or more. The brain changes were seen based on depth of planning, communication, problem solving and other and other more advanced cognitive functions. In the past 10,000 years and following the limited nutrition after agricultural flaws. Since childhood nutrition increased and disease declined, the brain size has increased once again. 

In conclusion, it can be argued that brain size is based on a variety of factors including genetic and societal. Hiwever, the past does not predict the future and with advancing technology and engineering we can understand and analyse the history of the brain much better and come into more accurate conclusions regarding the evolution f the mammalian brain. 

References:

1. Springer Science- https://link.springer.com/referenceworkentry/10.1007/978-3-540-29678-2_3146#:~:text=While%20the%20mammalian%20brain%20has,number%20of%20functionally%20distinct%20areas. 
2. Alzheimer’s disease- https://www.alz.org/alzheimers-dementia/what-is-alzheimers/brain_tour
3. NCBI- https://www.ncbi.nlm.nih.gov/books/NBK545206/#:~:text=The%20spinal%20cord%20divides%20into,exit%20through%20the%20intervertebral%20foramen. 
4. Science- https://www.science.org/doi/10.1126/sciadv.abe2101
5. Scientific American- https://www.scientificamerican.com/article/how-has-human-brain-evolved/