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Our knowledge concerning human evolution is apt to be limited and general. We’re unlikely to know the specifics about how our various body parts—eyes, nose, ears, tongue, hands, feet, knees, spine, and genitals—originated. We’re also probably curious about these matters; the need to know is basic to all of us, after all. This list will satisfy curiosity, even as it prompts questions to which scientists are, even now, themselves seeking answers.

Related: Top 10 Weirdest Products of Evolution

10 Eyes

The evolution of the eye is believed to have an origin common among cephalopods (carnivorous marine mollusks) and vertebrates, the latter of which, of course, include humans. The development of various types of eyes paralleled one another and was not original as such. Eyes were preceded by eye spots, photo-sensitive organelles composed of photoreceptor cell clusters and pigment cells.

As Scientific American explains, a billion years ago, multicellular organisms emerged from two groups: “one with… a top side and bottom side but no front or back” and the “other—which gave rise to most of the organisms we think of as animals—with left and right sides that are mirror images of one another and a head end.” The latter branch of animals, in turn, diverged around 600 million years ago as invertebrates and vertebrates appeared.

Further development during the Cambrian period produced the compound eye among all adult insects, spiders, and crustaceans, as well as the “camera-style eye” of squid, octopi, and humans. The photoreceptors on the marine mollusks’ eyes have the same kind of photoreceptors as those of insect eyes, whereas human eyes, like those of other vertebrates, contain cones for daylight vision and rods for nighttime vision.

As in the evolution of other anatomical parts, the demands of the organism’s environment led to specific adaptations that shaped an animal’s eye structure, appearance, function, and abilities, explaining the forward location of predators’ eyes, which aids their depth perception, versus the side placement of prey animals, which provides the latter’s superior field of vision.[1]

9 Nose

Hiroki Higashiyama of the University of Tokyo Graduate School of Medicine and his research team concluded some interesting facts about the evolution of the nose by tracking facial development in embryos of different species. They focused on embryonic cells that produce the physical structures of the face—”frontonasal prominence” cells develop into mammals’ protruding noses while their jaws form from a separate group of cells, the “maxillary prominence.”

Their discovery was borne out by their examination of fossils, with the jaws and noses of mammals indicating the occurrence of “transitional bone structures from the evolutionarily older reptile model to the more recently evolved mammalian structure.”

Of course, other environmental demands and challenges also account for the evolution of the nose, especially concerning the shapes of human noses. Tropical climate encouraged the development of wide, open nostrils and flat noses, while cold climes selected slender slits for nostrils and narrower noses. Climate’s effects on natural selection will continue to diminish now that heating and air conditioning have reduced the impact of hot and cold temperatures on noses’ continuing evolution, with “sexual selection” becoming more prominent in the future selection of noses.[2]

8 Ears

Scientists have not fully unraveled the mystery of the evolution of the ear. The fact that its growth pattern deviates from that of the remaining skeleton complicates the matter, as does the question of how this organ could have developed so that it provides a significant adaptation to the demands of life in aquatic, terrestrial, subterranean, and aerial environments. Although the enigma hasn’t been completely solved, the University of Vienna’s Philipp Mitteroecker and his team of researchers may have provided a piece of the puzzle.

The “evolutionary transformation of the primary jaw joint into the mammalian ear ossicles,” they believe, may have increased an independent adaptation of the different functional units of the ear despite the “tight spatial entanglement of functional ear components,” allowing a greater range of structural and functional diversity among organisms, the unique challenges of their respective environments notwithstanding.[3]

7 Tongue

The tongue, like other organs, has evolved to meet a variety of environmental challenges: Salamanders’ sticky tongues enable them to snag insects, snakes’ tongues to smell their surroundings, hummingbirds’ tongues to collect flower nectar, bats’ tongues to conduct echolocation, and humans’ tongues to eat, talk, kiss, and expand the brain capacity. However, the origin of these organs remains unsolved and hard to define since their structures, appearances, locations, and functions are extraordinarily diverse.

There are no hard-and-fast conclusions about how this amazing appendage came about, but evolutionary biologist Kurt Schwenk and functional morphologist Van Wassenbergh have a theory. In early land vertebrates, the “branchial arches and related muscles began to change to form a ‘prototongue,’ perhaps a muscular pad attached to the hyoid that flapped when the hyoid moved.”

Over time, the pad became longer, more controllable, and more adept at grabbing and maneuvering prey. These changes, in turn, might have been critical to the migration of some marine animals onto land by making it possible to ingest food without suction, which is how fish swallow their food.[4]

6 Hands

Scientists have long wondered about the origin of hands. Charles Darwin speculated that a common ancestor with limbs with digits explains the same pattern of construction evident in the mole’s clawed feet, the bat’s and bird’s wings, and the porpoise’s paddles. As John A. Long and Richard Cloutier point out in a Scientific American article, paleontology, genetics, and embryology increasingly supported his idea, showing that “the bones that make up the human hand are also found in frogs and birds and whales” and identifying some of the genes that control the development of hands and wings and flippers, among other variations.

The mystery of how the human hand and wrist evolved from fish fins remained unsolved until the discovery of an exceptional 375-million-year-old fossil of a fish, Elpistostege watsoni. This fossil provided evidence of bone structures similar to human fingers in its fins, indicating that digits evolved before vertebrates transitioned to land. Additionally, a 380-million-year-old Tiktaalik roseae fossil with well-developed arm bones, mobile wrist joints, and skull traits shared by tetrapods further supports this theory.[5]

5 Feet

The evolution of human feet and bipedal walking is a cornerstone of human evolution. Around 6-7 million years ago, early hominins began to adapt to bipedalism, a significant shift from the arboreal lifestyle of their ancestors. This transition involved changes in the pelvis, spine, legs, and feet.

Humans have unique toes that face upward, allowing for bipedal locomotion. The outside toes acquired upward-facing joints about 4.4 million years ago, while the big toe didn’t evolve until 2.2 million years later. The human foot evolved to have a robust heel, arches for shock absorption, and non-opposable big toes aligned with the other toes to aid forward propulsion. These adaptations allowed for efficient, long-distance walking and running, crucial for survival in varied environments.

Fossil evidence, such as the famous “Lucy” (Australopithecus afarensis), shows intermediate forms of bipedalism, while footprints in Laetoli, Tanzania, indicate fully developed bipedalism around 3.6 million years ago.

Also, the longitudinal and transverse arches of the foot give humans stiff feet, findings that may shape designs for prosthetic and robotic feet and allow innovations in podiatry, evolutionary biology, and robotics. [6]

4 Knees

Harvard correspondent Clea Simon refers to the human knee as a flawed masterpiece–, its osteoarthritis, its achievement, and its design. Walking upright places all the weight on the hips, knees, and ankles, so the switch from quadrupedalism to bipedalism caused changes in knee cells and shape within the limits of little variation. Only slight deviations occurred due to “genetic variants” following the expansion and drift of “human populations.”

These deviations and wear and tear on the knee over decades of life contribute to the onset of osteoarthritis later in life. As Simon sums up the results 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.”[7]

3 Spine

The evolution of the human spine played a critical role in the development of bipedalism. The spine underwent significant structural changes in early hominins to support upright walking. Unlike the relatively straight spine of quadrupedal primates, the human spine developed distinct curves: the cervical and lumbar lordosis and the thoracic kyphosis. These curves helped balance the body’s weight over the pelvis, providing the stability and flexibility necessary for bipedal locomotion.

The vertebrae became more robust to support the increased weight bearing on the spine. Additionally, the foramen magnum, the hole through which the spinal cord passes, shifted to a more central position under the skull. This repositioning facilitated an upright head posture, which is essential for bipedalism.

The pelvis also adapted, becoming shorter and broader, which altered muscle attachments and helped maintain balance. These spinal adaptations allowed for an efficient upright gait, reducing energy expenditure during long-distance walking and running. Fossil evidence from species such as Australopithecus afarensis demonstrates these spinal changes, highlighting the gradual transition to bipedalism that distinguished early humans from their ape ancestors.

Like the knee, as Elizabeth Pennisi points out in an online Science article, the spine’s evolution fueled both the rise of mammals—and human back problems. Various regions of the spine bear portions of the body’s weight, their burdens having been determined by the divergence of mammals and reptiles, which led to mammals’ “regionalized vertebral column,” explains paleontologist Christian Kammerer. The lumbar region allows mammals to do all sorts of different things, paleontologist Stephanie Pierce says, but there is also a downside, namely back pain.[8]

2 Penis

When humans’ prehistoric ancestors lived in the sea, females’ eggs floated upon the surface of the water, and males released sperm to fertilize the eggs. The migration of marine animals onto land precipitated the need for a new means of delivering sperm since fertilization would have to occur inside the female’s body to prevent the eggs from dehydrating.

The penis was nature’s solution, but how this organ evolved remained a mystery until Harvard Medical School researcher Patrick Tschopp and his colleagues started studying snakes and lizards, the former of which has not one, but two, penises, or hemipenises (also spelled “hemipenes”), to see whether the theory that the penis and limbs evolved around the same time is likely to be true, NPR Senior Editor and Correspondent Geoff Brumfiel explains.

It turns out that snakes lost their limbs but not their penises, the two hemipenes developing in place of the snake’s legs. Tschoop and his team also found that, in mice, a penis also grows from the tail region and that, in both snakes and mice, the digestive tract, “the most ancient part of any animal,” prompts the growth of the penis during embryological development, which, like the liver and pancreas, buds from the gut. Researchers are now in the process of seeking to learn whether the penis and the vagina coevolved.[9]

1 Female Reproductive Tract

While it has been determined that female genitalia can be “highly diverse in form and function both within and among species,” evidence also suggests that, yes, female genitalia can also coevolve with male genitalia. Among certain stalk-eyed flies, despite females’ highly divergent spermathecal ducts, these body parts have “coevolved with the genital process of males.” A female may be more attracted to a male because her genitalia favorably affects his ability to “engage his genitalia correctly for insemination and/or selectively storing and using sperm for fertilization.”

Females may be more attracted to males with genitalia that support their reproductive goals, leading to the natural selection of male genitals. An example indicating this tendency is cited. “Ecologically driven changes in the ovipositor have driven correlated changes in male genital traits, leading them to abandon the use of the hook-like parameres in order to achieve genital coupling in the presence of the novel elongated ovipositor in the females of this species.”

Such coevolution is likely to have happened in humans, as well, research by Patricia L. R. Brennan and Richard Prum concludes. Genital coevolution between the sexes is expected to be common, they propose, stating that, for one thing, natural selection, including female choice of mate and competition among males for female mates, allows “copulation to be mechanically possible.”[10]

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