'your inner fish' neil shubin chpts. 8-11

Chapter 8: Making Scents (The Evolution of Smell)

How Smell Works

• The ability to detect scents comes from olfactory receptor (OR) proteins found in the sensory neurons inside the nose.

• Each OR protein binds to a specific type of odor molecule, sending signals to the brain.

• The diversity of OR proteins allows animals to distinguish thousands of different smells.

The Genetics of Smell

• Humans have about 1,000 olfactory receptor genes, but nearly half are nonfunctional (pseudogenes)—they no longer produce working proteins.

• In contrast, mammals like dogs and mice have most of their olfactory genes still functional, giving them a far superior sense of smell.

• This suggests that humans have lost some reliance on smell over evolutionary time.

Olfaction Across Species

• Fish: They have olfactory genes that detect chemicals in water, but their receptors are different from land animals because water carries scents differently than air.

• Amphibians & Reptiles: Many have Jacobson’s organ (vomeronasal organ, VNO), which detects pheromones—chemicals used for communication. Snakes famously use it when flicking their tongues.

• Mammals: Many have both olfactory receptors and a functional VNO, but primates (including humans) have a reduced or nonfunctional VNO.

• Humans: As primates evolved better vision, their need for an acute sense of smell declined, leading to the loss of many olfactory genes.

Evolutionary Significance

• The shared structure of olfactory genes across species shows that the ability to detect odors evolved long before mammals or land animals appeared.

• Mutations and gene loss in primates indicate natural selection favors vision over smell in certain environments.

• The fact that humans still carry pseudogenes for smell reveals our evolutionary history—we have the genetic remnants of a highly sensitive olfactory system that was once essential.

Chapter 9: Vision (The Evolution of Eyes)

How Vision Works

• Light enters the eye, passing through the cornea and lens, which focus it onto the retina at the back of the eye.

• The retina contains photoreceptor cells:

  • Rods: Detect low light, black-and-white vision.

  • Cones: Detect color, require bright light.

• The optic nerve transmits signals from these cells to the brain.

Evolutionary Origins of Eyes

• The simplest form of light detection appears in single-celled organisms, which have light-sensitive proteins but no actual eyes.

• Flatworms have eye spots—clusters of light-sensitive cells that allow them to sense the direction of light.

• Mollusks (e.g., squid and octopuses) developed camera-like eyes, independently from vertebrates.

• Vertebrates' eyes evolved from a patch of light-sensitive cells that folded inward, forming a primitive lens and retina.

The Pax6 Gene: The Master Eye Switch

• A single gene, Pax6, controls eye development across species.

• When scientists inserted mouse Pax6 genes into fruit flies, the flies still developed normal fly eyes—not mouse eyes—suggesting that Pax6 is a universal eye-building gene.

• This discovery showed that the genetic blueprint for eyes is ancient, dating back to a common ancestor of insects, mollusks, and vertebrates.

Flaws in the Human Eye

• The vertebrate retina is wired backward—light must pass through layers of nerve cells before reaching the photoreceptors.

• This design creates a blind spot, which is absent in octopuses, whose eyes are better structured.

• This shows that eyes were not perfectly designed but rather shaped by evolutionary constraints.

The Evolution of Color Vision

• Most mammals see in two colors (dichromatic vision), but primates evolved a third type of cone, allowing for trichromatic vision (red, green, and blue).

• This adaptation was likely driven by the need to spot ripe fruit in trees.

Evolutionary Significance

• Eyes evolved multiple times independently, but they all share a common genetic foundation.

• Despite their complexity, eyes were not built from scratch but modified from simpler structures over millions of years.

Chapter 10: Ears (The Evolution of Hearing and Balance)

How Hearing Works

• Sound waves are funneled into the outer ear, causing the eardrum to vibrate.

• The vibrations pass through three tiny bones (ossicles) in the middle ear:

  • Malleus (hammer)

  • Incus (anvil)

  • Stapes (stirrup)

• The ossicles amplify the sound and transmit it to the cochlea, a fluid-filled structure in the inner ear that converts vibrations into nerve signals.

Evolution of the Ear from Fish to Humans

• Fish do not have eardrums or middle ear bones, but they detect vibrations in water through their lateral line system.

• Early land animals evolved a single middle ear bone (stapes) to amplify airborne sound.

• Reptiles and early mammals developed additional bones to improve hearing.

• In mammals, two jawbones from reptilian ancestors shrank and moved into the middle ear, forming the three-bone system unique to mammals.

Key Fossil Evidence

• Tiktaalik (375 million years ago) had a skull structure that suggests an early form of sound detection on land.

• Therapsids (mammal-like reptiles) had intermediate stages of jawbones evolving into ear bones.

• Fossils show how the reptilian jaw joint (articular and quadrate bones) transitioned into the mammalian ear ossicles (malleus and incus).

Evolutionary Significance

• The transformation of jawbones into ear bones is one of the best-documented examples of how structures can change function over time.

• The connection between our jaw and ear still exists in embryos—human fetuses briefly have jawbones that resemble those of reptiles before shifting into ear bones.

Chapter 11: The Meaning of It All

The Big Picture

• Our bodies are built from ancient structures, from fish-like embryos to reptilian jawbones.

• Evolution explains why our bodies have flaws—we are not perfectly designed but shaped by history.

Why Evolution Matters to Medicine

• Birth defects can be explained by developmental genes inherited from ancestors.

• Back pain and hernias result from our shift to bipedalism, which put stress on a skeleton originally adapted for walking on all fours.

• Tinnitus (ringing in the ears) is likely an evolutionary leftover from early sound detection systems.

Final Message

• Evolution is not just about fossils—it has real implications for human health, biology, and medicine.

• By understanding our evolutionary past, we can better understand and address issues in the present.