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Our understanding of human evolution is often broad and imprecise. We may never fully uncover the exact origins of many of our body parts—eyes, nose, ears, tongue, hands, feet, knees, spine, and genitals. Our innate curiosity about these origins is universal. This list will quench your curiosity, even as it raises more questions that scientists continue to explore.
10. Eyes
The evolution of the eye is thought to have originated from a common ancestor shared by cephalopods (marine mollusks) and vertebrates, including humans. The development of various eye types unfolded in parallel rather than being entirely unique. Before eyes, there were eye spots—light-sensitive clusters of photoreceptor cells surrounded by pigment cells.
As Scientific American describes, about a billion years ago, multicellular organisms arose from two distinct groups: one with a top and bottom but no clear front or back, and another—giving rise to most modern animals—with symmetrical left and right sides and a distinct head. This latter group split around 600 million years ago, leading to the appearance of invertebrates and vertebrates.
During the Cambrian period, the compound eye evolved in adult insects, spiders, and crustaceans, alongside the 'camera-style eye' found in squid, octopuses, and humans. The photoreceptors in the eyes of marine mollusks share similarities with those in insect eyes, while human eyes, like those of other vertebrates, contain cones for daylight vision and rods for seeing in the dark.
As with other anatomical features, the environment of an organism drove specific adaptations that influenced the structure, appearance, function, and capabilities of its eyes. For instance, predators typically have forward-facing eyes, aiding depth perception, whereas prey animals have eyes on the sides of their heads, enhancing their field of vision.
9. Nose
Hiroki Higashiyama from the University of Tokyo Graduate School of Medicine and his team uncovered fascinating insights into the evolution of the nose by studying facial development in the embryos of various species. They focused on the embryonic cells responsible for forming the facial features—'frontonasal prominence' cells create mammals' protruding noses, while jaw formation comes from a different set of cells known as 'maxillary prominence.'
Their findings were confirmed through fossil analysis, with the jaws and noses of mammals revealing transitional bone structures between the evolutionary models of reptiles and the more recent mammalian structure.
The evolution of the nose is influenced by various environmental pressures. In tropical regions, the wide, open nostrils and flat noses evolved to cope with the heat, while colder climates selected for narrower noses with thinner nostrils. However, as modern heating and air conditioning have lessened the impact of extreme temperatures, the natural selection based on climate is fading. Instead, 'sexual selection' is expected to become a more significant factor in shaping the future evolution of noses.
8. Ears
The mystery behind the evolution of the ear remains largely unsolved. Its growth pattern is distinct from the rest of the skeleton, adding complexity to the puzzle. Additionally, how the ear evolved to adapt to different environments such as aquatic, terrestrial, subterranean, and aerial ones remains unclear. While the exact answer eludes us, researchers led by Philipp Mitteroecker from the University of Vienna might have uncovered part of the solution.
Mitteroecker and his team believe that the 'evolutionary transformation of the primary jaw joint into the mammalian ear ossicles' may have played a key role in the ear's adaptation. This transformation, despite the tight interconnection of the ear's components, might have allowed for a more varied range of structural and functional diversity among organisms, tailoring the ear to the unique demands of different environments.
7. Tongue
The tongue, like many other organs, has evolved to serve a wide array of environmental needs. Salamanders' sticky tongues help them catch insects, snakes use theirs to detect their environment, hummingbirds rely on their tongues to gather nectar from flowers, bats' tongues aid in echolocation, and humans use their tongues for eating, speaking, kissing, and enhancing brain function. However, the precise origin of this versatile organ remains a mystery, as its forms, locations, and functions vary greatly across species.
Though there are no definitive answers about the tongue's origins, evolutionary biologists Kurt Schwenk and functional morphologist Van Wassenbergh propose a theory. They suggest that in early land vertebrates, the 'branchial arches and related muscles began to evolve into a 'prototongue,' potentially a muscular pad attached to the hyoid that would flap when the hyoid moved.
Over time, this pad became longer, more maneuverable, and better at capturing and handling prey. Such changes were crucial for the transition of some marine animals onto land, as they allowed for the ingestion of food without the need for suction, which is how fish typically swallow their meals.
6. Hands
The origin of hands has long puzzled scientists. Charles Darwin speculated that a common ancestor with limbed digits might explain the similar skeletal patterns seen in the claws of moles, the wings of bats and birds, and the paddles of porpoises. As noted by John A. Long and Richard Cloutier in a Scientific American article, findings from paleontology, genetics, and embryology have increasingly supported Darwin’s theory. They showed that 'the bones that make up the human hand are also found in frogs and birds and whales,' and identified some of the genes responsible for developing hands, wings, and flippers, among other limbs.
The mystery surrounding the evolution of the human hand and wrist from fish fins was solved with the discovery of a remarkable 375-million-year-old fish fossil, Elpistostege watsoni. This fossil revealed bone structures in its fins that resembled human fingers, providing evidence that digits developed before vertebrates made their transition to land. Further support for this theory comes from a 380-million-year-old Tiktaalik roseae fossil, which shows well-developed arm bones, mobile wrist joints, and skull features characteristic of tetrapods.
5. Feet
The evolution of human feet and the ability to walk upright is a crucial chapter in human evolution. Approximately 6 to 7 million years ago, early hominins began to adapt to bipedalism, marking a significant shift away from their tree-dwelling ancestors. This transition involved modifications to the pelvis, spine, legs, and feet.
Humans have evolved unique toes that point upward, a key adaptation for bipedal locomotion. Around 4.4 million years ago, the outer toes developed upward-facing joints, while the big toe didn't acquire its modern function until 2.2 million years later. The human foot became designed for walking and running over long distances, with a sturdy heel, shock-absorbing arches, and a non-opposable big toe that aligns with the other toes to enhance forward motion. These features proved vital for survival across diverse environments.
Fossil discoveries, including the famous 'Lucy' (Australopithecus afarensis), show early stages of bipedalism, while footprints discovered in Laetoli, Tanzania, reveal fully developed bipedalism as far back as 3.6 million years ago.
The longitudinal and transverse arches of the foot give humans rigid feet, a characteristic that may influence the design of prosthetic and robotic feet, offering opportunities for breakthroughs in podiatry, evolutionary biology, and robotics.
4. Knees
Harvard correspondent Clea Simon calls the human knee a 'flawed masterpiece'—noting its osteoarthritis, its remarkable function, and its design. The shift to walking upright places the entire weight of the body on the hips, knees, and ankles. Consequently, the transition from quadrupedalism to bipedalism resulted in modifications to the cells and shape of the knee, with minimal variation. Only slight deviations occurred due to 'genetic variants' as human populations expanded and drifted.
These changes, combined with the wear and tear accumulated on the knee throughout a lifetime, contribute to the development of osteoarthritis later in life. As Simon aptly puts it about the outcomes of evolution’s 'flawed masterpiece,' 'The stiffness and soreness humans feel today may simply have piggybacked on an evolutionary advantage. In other words, osteoarthritis came along with the knee.'
3. Female Reproductive Tract
It has been established that female genitalia can vary greatly in form and function, both within and across species. Furthermore, there is evidence suggesting that female genitalia may coevolve with male genitalia. In certain stalk-eyed flies, despite significant differences in the structure of the female spermathecal ducts, these reproductive organs have evolved together with the male genital process. This coevolution may cause females to be more attracted to males whose genitalia facilitate proper insemination or sperm storage for fertilization.
Females might be more drawn to males whose genitalia align with their reproductive goals, resulting in the natural selection of male genital traits. One example of this phenomenon is the change in the female ovipositor, which drove related changes in male genital traits. Specifically, males abandoned the use of hook-like parameres to enable genital coupling with females that had a newly elongated ovipositor.
This type of coevolution likely occurred in humans as well, according to research by Patricia L. R. Brennan and Richard Prum. They suggest that genital coevolution between the sexes is common, arguing that natural selection—along with female mate choice and male competition—ensures that copulation is mechanically feasible.
2. Penis
When humans' ancient ancestors lived in the sea, females’ eggs would float on the water’s surface, and males released sperm to fertilize them externally. However, as marine animals moved onto land, a new method for sperm delivery became necessary, as fertilization had to occur within the female's body to prevent the eggs from dehydrating.
The penis was nature’s answer to reproduction, but its evolutionary origin remained unclear until Harvard Medical School researcher Patrick Tschopp and his team began studying snakes and lizards. Notably, snakes possess two penises, or hemipenes, which provided a unique opportunity to test the theory that the evolution of the penis and limbs happened simultaneously, as explained by NPR Senior Editor Geoff Brumfiel.
It turns out that while snakes lost their limbs, they retained their penises. The two hemipenes evolved in place of the snake’s legs. Tschopp and his team also discovered that in mice, the penis grows from the tail region, and that in both snakes and mice, the most ancient part of any animal—the digestive tract—triggers the development of the penis during embryonic growth, much like how the liver and pancreas emerge from the gut. Researchers are now exploring whether the penis and vagina coevolved.
1. Spine
The evolution of the human spine was pivotal in the emergence of bipedalism. Early hominins’ spines underwent significant changes to accommodate upright walking. Unlike the relatively straight spines of quadrupedal primates, the human spine developed distinctive curves: cervical and lumbar lordosis, as well as thoracic kyphosis. These curves played a crucial role in balancing the body's weight over the pelvis, providing the necessary stability and flexibility for bipedal movement.
The vertebrae became stronger to handle the increased weight load on the spine. Furthermore, the foramen magnum, the opening through which the spinal cord passes, shifted to a more central position beneath the skull, facilitating an upright posture, which is essential for bipedalism.
The pelvis underwent significant changes, becoming shorter and broader, which in turn affected muscle attachments and helped maintain balance. These spinal changes were key to efficient bipedal movement, allowing early humans to walk and run long distances with reduced energy expenditure. Fossil evidence, such as that from *Australopithecus afarensis*, shows these spinal adjustments, marking the gradual shift to bipedalism that set early humans apart from their ape ancestors.
Similar to the knee, the evolution of the spine played a crucial role in both the development of mammals and the emergence of human back problems. As noted by Elizabeth Pennisi in an online *Science* article, the way the spine bears the body's weight was shaped by the evolutionary split between mammals and reptiles. This divergence led to mammals having a 'regionalized vertebral column,' explains paleontologist Christian Kammerer. The lumbar region allows for a wide range of movement, says paleontologist Stephanie Pierce, but this flexibility also comes with the downside of back pain.
