Your Inner Fish: Episode 1 - From Water to Land

Exam Logistics & Class Schedule

Exam Sign-up Resolution: The issue with students being asked for a redundant exam duration during registration has been resolved.
Early Sign-up Encouraged: Students were advised for weeks to sign up early for the exam via RegisterBlast.
Exam Format: The exam is online and must be taken at a test center. Test centers provide the password to access the exam; it cannot be opened independently.
Class on the 29th: There will be no formal class meeting on the 29th (Wednesday).

Video Series Introduction

Video Content: Today's class features the first in a three-part video series titled "Your Inner Fish."
Narrator & Focus: The series is narrated by Neil Shubin, a researcher, with the first episode focusing on his personal discoveries.
Video Duration: The video is approximately 53 minutes long.
Technical Check: Students were instructed to confirm audio functionality once the video started.
Post-Video Instruction: Students must remain after the video for a short summary lecture and discussion by the professor; they should not leave immediately after the video concludes.
Exam Relevance: The video content, along with the subsequent two episodes, will be on the exam. An accompanying document with suggested study questions will be provided to guide preparation.

"Your Inner Fish" Episode 1: Summary

Core Concept: Human Bodies as a Product of Evolutionary History

Shubin's Perspective: Neil Shubin, a fish paleontologist, views human bodies through an evolutionary lens, seeing "ghosts of animals past" and "glimpses of an epic story that's hidden inside us all."
Ancestral Connections: Human features like the ability to grip (from primate ancestors) and advanced hearing (from shrew-sized creatures) trace back millions of years.
Goal of the Search: To reveal why humans look the way they do by exploring distant ancestors, particularly fish that transitioned to land hundreds of millions of years ago, which are crucial for understanding human necks, lungs, limbs, and hands.

The Search for Transitional Fossils

Setting the Stage: Shubin describes his journey to the desolate Canadian Arctic, a "treasure" trove of ancient rock fossils, specifically fish that hold clues to human body development.
Paleontologist as Anatomy Chairman: Shubin's role at the University of Chicago highlights how fish paleontology offers robust "road maps" for teaching human anatomy, as similarities are often found in other creatures.
Shared Anatomy with Fish: Humans are related to fish, evidenced by shared fundamental anatomy: bony skeletons, backbones, and skulls, first appearing in fish.
The Tree of Life: This shared ancestry is illustrated by tracing life's history: microscopic organisms (billions of years ago)
ightarrow fish (400 million years ago)
ightarrow amphibians (360 million years ago)
ightarrow reptiles
ightarrow first mammals (200 million years ago)
ightarrow primates. All modern reptiles, birds, and mammals (including humans) are descended from ancient fish.

Human Anatomy: Nerves and the Hand

Nerve Complexity: The intricate neural network in the human head is fundamentally similar to the basic wiring found in fish.
The Human Hand: Shubin expresses awe at the hand's intricate connections (bone, tendon, muscle), enabling fine movements. The fine muscles controlling finger motion are described as "quintessentially primate and human."

The "One Bone, Two Bones, Little Bones, Digits" Pattern

Sir Richard Owen's Observation: In the 19th century, anatomist Richard Owen noted a consistent skeletal pattern in the limbs of all four-legged animals: a single proximal bone, followed by two distal bones, then a cluster of smaller bones, and finally the digits (fingers/toes).
- Examples: Dogs (running), birds (wings) exhibit this pattern – one bone, two bones, many little bones, digits, albeit modified for different functions.
Charles Darwin's Explanation: Darwin proposed that this common pattern in diverse animals indicates a shared common ancestor that also possessed a version of this limb structure, providing an evolutionary explanation for Owen's mystery.
Tracing the Pattern: This pattern can be traced back through primates, early mammals, and the earliest tetrapods (four-limbed animals).

The Great Mystery: Fish to Limbs

The Evolutionary Gap: At around 400 million years ago, the fossil record showed a stark transition from prehistoric fish with fins to land animals with limbs, leaving a significant gap.
Darwin's Prediction: Darwin hypothesized the existence of "transitional forms" – ancient animals bridging this gap, potentially possessing both limb and fin features.

The Search for the Transitional Fossil

Methodology: Shubin and his team (with geologist Ted Deschler) looked for rocks of the right age (Devonian era, around 360 million years old) and type (those likely to hold fossils).
Pennsylvania Discoveries: Initial searches in Pennsylvania (e.g., Red Hill, often exposed by road construction) yielded Hynerpeton, an early limbed animal (tetrapod shoulder girdle), but still too far along the evolutionary path.
Ichthyostega: Mentions Jenny Clack's work in Greenland, discovering Ichthyostega, an early tetrapod with forelimbs capable of lifting the upper body on land, but hind limbs still paddle-like, suggesting it was "right at the edge" of water-to-land transition. Still, a gap remained between Ichthyostega and ancient fish.
Arctic Expedition: A geological map revealed a strip of Devonian rocks in Northern Canada, 10 million years older and previously unsearched. This became the new target.
- Challenges: The Arctic presented extreme conditions: no human settlements, severe weather, polar bears, and a narrow window (July) for fieldwork.
- Ancient Environment: Despite the frozen present, the area was a warm flood plain 375 million years ago, filled with diverse life in rivers and swamps.

Embryology: A Window into Evolutionary History

Embryonic Similarities: Shubin's microscope work on embryos revealed that all animals start as single cells, gradually forming a body. Early fish and human embryos are "almost identical," sharing a basic body plan (head, body, tail, pharyngeal area).
Gill Arches: Both fish and human embryos develop gill arches (swellings in the pharyngeal area). In fish, these form gill apparatus; in humans, they contribute to the lower jaw, middle ear, and voice box.
"Your Inner Fish Can Come Out": Sometimes developmental processes go awry, revealing this ancestral past. The example of Molly Richardson, born with a gill pit (branchial cleft cyst), is a "leftover from an ancient gill."
Repurposed Structures: Many human muscles, nerves, and bones used for speaking and hearing correspond to fish gill structures.
Testicles and Hernias:
- Fish Gonads: Located near the chest/heart.
- Mammalian Gonads: Start deep in the body, like fish, but descend to a cooler external location (scrotum) because mammalian sperm is heat-sensitive.
- Evolutionary Flaw: This descent creates a "weak spot" in the body wall, making human males susceptible to hernias. This is a consequence of evolution repurposing an ancient body plan – humans are "jerry-rigged fish."

The Discovery of Tiktaalik

Second Arctic Season (2000): After widening the search and facing dangers, a team member, Jason, discovered a site with thousands of fossil fish bones, indicating a rich ancient riverbed.
Locating the Source Layer: It took weeks to find the specific rock layer from which the fossils emanated, identified by minute white flecks of scale and bone.
The Breakthrough (2004): During the July 2004 season, the team found a flat-headed fish snout protruding from the rock, a key indicator of a transitional form.
Naming: The local Inuit people named the fossil Tiktaalik, meaning "large freshwater fish."
Anatomical Mixture: Tiktaalik indeed proved to be Darwin's predicted transitional form, an "anatomical mixture":
- Fish-like features: Scales, fins, gills.
- Tetrapod-like features: Lungs for air breathing, and crucially, a neck (the earliest ever found, allowing head movement), and within its fins, the underlying "one bone, two bones, lots of bones" pattern, including a primitive "wrist."
Functional Implications: Tiktaalik could use its neck to spot predators/prey and its strong fins to lift its body out of the water, effectively performing a "push-up," thus opening up a "whole new frontier" for life on land.

The Power of Genetics: Sonic Hedgehog

Genetic Revolution: Evolutionary biology saw a revolution in understanding the genetic link between fins and limbs.
Cliff Tabin's Work on Chick Embryos: Geneticist Cliff Tabin at Harvard studied digit formation in chicken limb buds.
John Saunders' Experiment (1950s): Transplanting a small patch of cells from one side of a chick limb bud to the other resulted in a second, mirror-image set of digits, indicating these cells produce a "long-range signal."
The Hedgehog Gene: Inspired by the hedgehog gene in fruit flies (which sends organizing signals based on proximity), Cliff Tabin's team searched for a chicken equivalent.
Discovery of Sonic Hedgehog: They found a gene, dubbed sonic hedgehog (after the video game character), active in the same special patch of cells identified by Saunders.
Confirmation Experiment: Implanting a bead containing pure sonic hedgehog protein onto the "wrong side" of a limb bud replicated Saunders' mirror-image digits, confirming sonic hedgehog as the key signal for digit patterning.
Role in Human Development: Sonic hedgehog shapes not just chicken wings but also mouse paws and human hands.
Polydactyly in Humans: The Hubbard family's son, Kamani, has six fingers and toes (polydactyly or heptactyly, though he had six). This condition often results from mutations that alter the sonic hedgehog gene's effect, causing it to produce too much protein, increasing its concentration, and leading to extra, fully functional digits.
Concentration Gradient: Sonic hedgehog creates a concentration gradient: strongest signal forms the pinky finger; as the signal weakens further away, it triggers the formation of other fingers sequentially, ending with the thumb.
Tracing Back to Fish: Postdoc Randy Don's research showed that sonic hedgehog also patterns the fins of ancient fish like skates. This gene's fundamental patterning function has been conserved for 400 million years, linking human hands to fish fins directly through a shared genetic mechanism.
Evolutionary Story: The history is embedded in our bones, muscle, and DNA, connecting us to our "inner fish." The transition from water to land set the stage for new anatomical inventions that form the core of human existence.

Post-Video Summary Lecture & Reinforcement

Homology: The video reinforced the concept of homology (similar structures due to common ancestry), demonstrating the lineage from fish to humans.
Richard Owen vs. Charles Darwin: Owen observed patterns (the "one bone, two bones, small bones, fingers"), but Darwin provided the evolutionary explanation of common ancestry.
Tetrapods: The term "tetrapod" (Greek: tetra = four, poda = limb) defines all four-limbed animals, from amphibians onward, including birds (whose forelimbs became wings).
The Missing Link: Tiktaalik filled the crucial morphological gap between aquatic fish and land-based amphibians.
Lobe-finned Fish Ancestry: The lecture highlighted lobe-finned fish (like coelacanths, lungfish, and euthenopterin, shown in the video) as key ancestors, differentiating them from ray-finned fish. Lobe-finned fish possess a distinct bony, fleshy lobe at the base of their fins, which contains bone structures homologous to early tetrapod limbs.
Evolution of Respiratory Structures: The evolutionary tree also shows changes in respiratory structures, from gills to lungs.
Bone Structure Comparison: Detailed comparison from perch (ray-finned, simple rays) to coelacanth (more bone in lobe, but not radius/ulna) to euthenopterin (humerus, early radius/ulna) to Acanthostega (humerus, radius, ulna, 8 digits, finned tail – a water-dwelling tetrapod) demonstrates the gradual progression, with Tiktaalik fitting perfectly between euthenopterin and Acanthostega.
Tiktaalik's Significance: Its wrist, mobile elbow, and robust muscle attachments demonstrate its capability for weight-bearing movement (like a "push-up"), marking a critical step towards terrestrial locomotion.
Reduction in Digits (Pentadactyly): Early tetrapods like Acanthostega had more digits (e.g., 8). Over evolutionary time, there was a refinement and reduction to the pentadactyl (five-digit) pattern seen in most modern tetrapods, including humans. Acanthostega itself exhibited more digits (8) than later ancestors, which had fewer (e.g., Tulerpeton with 6 digits) before the eventual five-digit pattern.
Genes as Master Switches: Genes, like sonic hedgehog, act as "master switches" rather than architectural blueprints, guiding the development of structures through mechanisms like protein concentration gradients.
Available Resources for Students: The video is available on Panopto. A handout with specific questions related to the video is provided; these questions directly reflect what will be on the exam to aid student preparation.