The Human Biological and Skeletal Heritage
The Earth is home to billions of living organisms, both extinct and living. The organisms that exist on Earth range from simple single-celled organisms to cetaceans larger than a school bus. Earth is home to vast diversity of creatures and body plans. The diversity of body plans employed by the organisms on earth is possible because of the process of evolution. Evolution is when traits inherited from parent to offspring over generations change over time, and on the evolutionary time scale eventually lead to the birth of new species. Natural selection is the mechanism that drives the machine of evolution. Simply put whatever trait that can help a creature survive and reproduce will be selected for, and any trait that hinders survival and reproduction will be selected against.
“When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.” The term “survival of the fittest simply refers to the fittest creature being the one best able to survive and reproduce. Humans are currently the fittest species on Earth being blessed with a highly developed brain and sentience. While human seem entirely unique and segregated from the rest of Earth’s organisms a closer look at their skeletons will show that humans are very similar to other creatures. Humans carry a skeletal and organic heritage that is part of a continuous narrative over the history of life.
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During the Hadean Eon the Earth was a ball of molten rock and hazardous gasses. This environment was hostile to the formation of life. The Hadean Eon ended when the Earth cooled enough to support ancient oceans and continents it had entered the Archean Eon. Between 4 to 3.8 billion years ago life began on Earth. The first life forms are believed to have been born deep in the oceans. Hydrothermal vents also called “black smokers” are specifically thought to be the nurseries where the first organisms emerged. Black smokers are under water geysers where hot water saturated with minerals spew out from deep beneath the earth. This mineral rich environment would have supplied the necessary minerals to form early organisms and provided them with the nutrients they need to survive.
The first organisms were simple single-celled organisms. These early life forms did not have a mitochondria, developed organelles, or a robust cell wall. They would have lived solitary lives without interacting with their neighbors. They were likely chemosynthetic feeders, using the sulfur in the water as food source. While simple these organisms reproduced quickly and some are likely to have been carried by currents to place all around the globe. Unfortunately fossil evidence of these early bacteria has not been found and as such much of what is known about these creatures comes from observation of modern-day chemotrophs and extrapolation of what Earths geologic conditions were like at the time.
The oldest fossils found by researchers are called stromatolites, these fossils contain the remains of bacteria similar to modern-day Cyanobacteria. The bacteria found in stromatolites are thought to be the first to migrate to shallow water and the first to use photosynthesis. “The first Photosynthesizers converted carbon dioxide to sugar by adding hydrogen from hydrogen sulfides. Scientists know this due to the yellow deposits of sulfur this process left behind in the rock.”
Later generations of this Bacteria would improve at photosynthesis by using the hydrogen found in water for their dietary needs. The innovation of photosynthesis is important because until that point Earth’s atmosphere had almost no oxygen and it was these Photosynthesizers that changed the atmosphere to include oxygen needed by other creatures to survive.
After bacteria evolved photosynthesis another group of bacteria evolved the ability to passively absorb nutrients and other unlucky bacteria around them for food. After many generations the predatory bacteria adapted from passively absorbing their food and evolved the ability to actively hunt other bacteria for food. Heterotrophs are the next innovation on the evolutionary time scale and their adaptations are important for several reasons.
First, heterotrophs established the first food chain which is important for the circulation of nutrients throughout the ecosystem. Secondly, the competition between heterotrophs and autotrophs accelerated the evolution of new adaptations by acting as another selection pressure. Thirdly, heterotrophs would go on to form symbiotic relationships with multicellular organisms millions of years later.
The next milestone in the organic narrative is the evolution of eukaryotic cells. Up until 2.7 billion years ago all the bacteria that existed were prokaryotic. These bacteria did not have organelles, a portioned nucleolus, or complex cell membranes. prokaryotic cells are also miniscule compared to eukaryotic cells, even small enough to fit inside of eukaryotes. Eukaryotes are believed to have evolved by swallowing smaller cells but not digesting them electing instead to form a symbiotic relationship with their prisoner. the host cell provides protection and food for the captured cell and in return the prisoner will perform tasks for the host.
Over many generations the captured cells became a permanent addition to the cell. In autotrophic cells this allowed for the formation of chloroplasts which made photosynthesis more productive and efficient. In heterotrophic cells this adaptation led to the formation of the mitochondrion. Millions of years later the photosynthetic cells would evolve to become the world’s plants and algae and the heterotrophic cells would evolve into the world’s animals.
When a simple cell multiplies it sometimes makes a mistake writing the new set of DNA, these mistakes are called mutations. Mutations are important because they are one of the ways of promoting genetic diversity in a species of bacteria. Occasionally when two simple bacteria interact they swap a few small sample of DNA with each other which also promotes genetic diversity. The next milestone in the evolutionary timeline is the development of sexual reproduction by eukaryotic cells.
Eukaryotic cells have a complex DNA structure wound up and encased in a nucleus as such they cannot exchange DNA with other cells the same way that prokaryotic cells can. Instead when eukaryotic cells reproduce they first unwind their DNA that was in the nucleus. The cell’s DNA is divided into two identical copies of the same pairs chromosomes. Then the strands of DNA temporarily stick together where they swap chromosomal pairs making a new and unique genetic combination.
Afterwards the two DNA strands separate and the parent cell divides into daughter cells that go on to seek and combine with the daughter cells of a different parent cell. The resulting offspring from this process is unique and different from both its parent cells. Sexual reproduction is important for this narrative because all the descendants of eukaryotic cells use some version of this method of reproduction whether they be plants, animals, or fungi.
The next important biological adaptation to evolve leading to humans was multicellularity. Multicellularity evolved when certain types of single-celled eukaryotic organisms began to live together in colonies. After many generations some of the cells in these colonies began to specialize and perform unique tasks. They started doing this because dividing the labor of the colony brought more rewards to the colony and its participants.
As communication between individual neighboring cells improved the colonies got more and more complex and over many generations of natural selection the cell colonies became the first multicellular organisms. Humans are multicellular organisms so it stands to reason that they are related to the first multicellular organism. The first multicellular organisms were likely soft-bodied sea sponges. These sea sponges were about as simple as an animal can be.
Their cells were not very differentiated, they did not have any organs, they have no skeleton, their nervous system was very primitive, and they had a simple body plan. Another creature that is debated to be as ancient as the sea sponge is the jelly fish. The reason for this debate is because jellyfish fit all the previously listed criteria for the first multicellular organism and because promising jellyfish fossils (figure 1) have been found that further support that claim.
The next adaptation to evolve that has contributed to skeletal and biological heritage of humans is the divergence of the sexes. Sexuality started with microbes that could only produce of spring with other microbes that are chemically compatible with them. After that, multicellular organisms took that adaptation further by evolving males and females.
The biological role of males is to provide sperm, which makes up half of the offspring’s DNA and the female’s role is to provide the egg which contains the other half of the DNA for the offspring. females usually bear the highest risk in terms of the cost of reproduction while males only have to provide sperm which is relatively low-cost. This is important for humans because this has informed the way humans reproduce and has led to skeletal changes reflecting the unique roles of the sexes. Such as a wider pelvis to accommodate child-birth in females compared to males. Also, the large human head led to a wider pelvis in humans compared to apes and hominins (figure 2).
The next major adaptation that led to humans to evolve was the development of the brain and complex nervous system. As animals gained mobility and the competition for resources and mates intensified, their nervous systems evolved to take in more and more stimuli and their brains became bigger and more complex. These upgraded brains allowed creatures to perform more complex tasks. One consequence of the improved nervous system is that it altered the body plans of creatures in uniform and lasting ways. The first important change to the body plan of animals to accommodate the improved nervous system is the development of the head.
Animals grouped their sensory organs such as their eyes, nose, ears and mouth in the head because that was the part of the body that was most likely to encounter important information. The next adaptation to happen to the body plans of animals was bilateral symmetry. The reason bilateral symmetry evolved was because of the development of the nerve cord. This bundle of nerves started in the head and traveled down the center line of the animal’s body. This nerve cord facilitated the travel of signals to and from the brain. The Cambrian arthropod found in the burgess shale (figure 3) displays a great example of early bilateral symmetry in the way its opposite limbs correspond to each other.
Narratively speaking all the creatures covered until this point did not have an internal skeleton. This changed with the evolution of the spine which had profound impact on the evolution of life and the body plan of humans. The spine is important because it gives support and rigidity for the animal allowing it to grow bigger and become stronger, faster, and more agile. The first spine to evolve is called the notochord, this spine was made of tough tissue and ran down the body of early fish that swam through the ocean.
After many generations these fish developed cartilaginous skeletons. These fish grew bigger than their non-vertebrate contemporaries and went on to dominate the food chain, the most successful creature with a cartilaginous skeleton is the shark like the one in (figure 4). This shark fossil is millions of years old and demonstrates that the skeleton is an important and enduring adaptation.
The next important adaptation in the narrative history of humanities skeletal history is the evolution of bones. For animals to develop the ability to walk on land one of the crucial things they need is a skeleton sturdy enough to support their weight. Originally fish covered their bodies in bony armor to protect them. The next improvement in this adaptation is when these fish began to use bone instead of cartilage for their skeletons.
Bones have many advantages over cartilage, first bones are stronger than cartilage allowing animals to grow even bigger than ever before. Secondly, bones are much harder than cartilage allowing them to give more shape to organisms. Lastly, bone offers better protection for the internal organs from blunt force trauma better than cartilage can. An extreme example of the strength of bones is the extinct giant sloth that was bigger than a car (figure 5). Without bones animals would not have been able to move to land.
The next big step in the evolutionary heritage for humanity is when lobed finned fish left the water and evolved into Tetrapods. Lobed finned fish are a type of bony fish that swam in the ocean millions of years ago. A species of lobed finned fish is still alive today called the coelacanth. This type of fish is important for the evolutionary narrative of humans because its fins are what became the limbs of all Tetrapods. Lobe finned fish lived in areas close to shore and adapted their swim bladders to let them breathe air. Then while on land their fins evolved to have fingers, toes, wrists, ankles, forearms, lower legs, upper legs, upper arms, and primitive shoulders and hips.
The evolution of Tetrapods led to the birth of amphibians, reptiles, birds, and the evolution of mammals with the last one being integral for the narrative of human skeletal heritage. Tetrapods are characterized by having four limbs, the ability to breath air and a bony skeleton. The first Tetrapods were very fish spending most of their time in or near the water. With time some of them evolved into amphibians.
Then they evolved into reptiles which where exothermic like amphibians but did not need to live in or near water to survive, allowing them to expand their range and diversity. After the evolution of reptiles and dinosaurs came the evolution of birds which adapted their front legs into wings and were endothermic able to regulate their own body temperature. The last descendants of Tetrapods to evolve are mammals, this however does not mean that mammals are descendants of birds, it just means that mammal evolved after birds.
The evolution of mammals is important for the skeletal and biological heritage of humans because humans are in fact mammals. Mammals are distinct from other classes for many reasons. First mammals are endothermic regulating their own body temperature. Mammals give birth to live young and they nurse those young with mammary glands that they are named after. Mammals are also typically covered in fur.
Figure 6 shows the skeleton of what an early mammal looked like. The body plan in this skeleton (figure 6) has all the skeletal features that the incredible diversity of mammal will take advantage of. The same long finger bones that make up a bat’s wings (figure 7) are the same bones in a whale’s flippers, a horse’s hooves, the paws of a lion, and the hands of an orangutan. As mammals, humans are quite anatomically unique but still fit into the mammalian mold. Humans have some hair on their bodies, they give birth to live young, they nurse those young, and they regulate their temperature internally.
Primates are the order that humans belong to, as such many of the unique skeletal traits attributed to them are found in humans in an easily recognizable way. Primates evolved hands with long and dexterous fingers along with opposable thumbs capable of performing delicate tasks. Primates also began to stand upright and walk similar to how humans walk for short distances, however this was not true bipedalism.
The primates most closely related to humans did not have tails, this adaptation was inherited by humans evident by the existence of the tail bone. While the primates also do not have the same vocal dexterity that humans have, this is because the shape of a primate’s mouth, neck, and skull cannot accommodate the vocal chords necessary for speech. Arguably the biggest difference between humans and other primates is the size and development of the brain of humans compared to other species.
Hominids are the family that humans are descendant from. Hominids are also called great apes, these animals are known for being larger than monkeys, having a complex social structure, incredible intelligence and creativity. Non-human Examples of great apes are orangutans, chimpanzees, and gorillas. Great apes have very large and well-developed brains. These large brains gave great apes a distinct advantage over their rivals. Their large brains allowed them to learn very quickly and be creative inventing new ways of gathering resources and other tasks related to survival.
Hominins are the sub family that humans belong to, the adaptations pioneered by these ancestors include bipedalism, speech, high intelligence, self-awareness and possibly speech. One difference that separates Hominins from great apes is that hominins are fully bipedal. The ability to walk up right allowed hominis to occupy a wider range of territory. However the feet of the hominins is different because they have an opposable big toe similar to a thumb. Humans however, have small toes not designed for grabbing but for walking long distances. It was also the more advanced hominins that began experimenting with early language.
This was possible for two reasons the first is that hominins were the first to have brains sophisticated enough to formulate language and secondly the structure of the hominin mouth would have been able to accommodate complex speech. Also like humans hominins are thought to have had a complex social structures and early tool use. Despite all these similarities humans and hominins look vastly different. Hominins range from being very short to slightly taller than humans and they had significantly more body hair than humans like the Neanderthal referenced in (figure 8). Neanderthals are the closest relatives to humans and they frequently interbred with humans before they went extinct. This means that humans are the last species left in the genus Homo.
Humans have inherited many adaptations from their ancestors. From primates, humans inherited the basic skeletal structure that would be improved upon by great apes and hominins. Humans carry the heritage bequeathed to them by bony fish and lope finned fish found in their bones and limbs. Tetrapods are the ancestors of mammals, birds, reptiles, and amphibians. This means that humans also have the basic body structure pioneered by them when they first made landfall from the seas. Microscopic organisms built the basic cellular structure that humans are built upon and use.
The first vertebrates made strides that allowed humans and other creatures to support their own weight on land. Humans also made their own evolutionary innovations. Humans have complex speech that comes from an intelligent brain. Humans also have the most intelligent brain in the entire animal kingdom. Humans are not separate and above the rest of the animal kingdom and the processes that affect them. Humans are part of a biological and skeletal heritage that forms a continuous narrative from the beginning of life to today.