Chordates and the Evolution of Vertebrates

Chordates and Vertebrates

All vertebrate animals are classified as chordates, sharing a common ancestry.
To be classified as a chordate, an animal must possess four major structures at some point during its development:
- Dorsal hollow nerve cord: This develops into the brain and spinal cord.
- Notochord: The structure that becomes the bones of the backbone, or vertebrae.
- Pharyngeal slits: These structures develop into lungs and the mouth in humans, and gills in fish.
- Post-anal tail: A tail that extends beyond the anus, present at some point in development. In humans, it reduces to the tailbone.

Embryonic Development

Embryos of different vertebrate species (human, chicken, tortoise, salamander, fish) appear very similar in early stages.
This similarity in embryos indicates a common ancestry.
The development of embryos is a complex field, with proteins involved having unusual names (e.g., sonic hedgehog, pikachurin).

First Vertebrates

The first vertebrates appeared in the ocean around 540 million years ago during the Cambrian period.

Vertebrate Characteristics

Vertebrate animals possess endoskeletons, a cranium (skull), and a backbone.
Jawless fishes (hagfish and lamprey) are exceptions as they lack jaws.

Hagfish

Hagfish are scavengers that live on the ocean floor.
They are known for producing a protein-based slime that expands and becomes very slippery in water.
Researchers are exploring the use of hagfish slime as a lubricant for machinery.

Lampreys

Lampreys are parasitic and attach to other fish (e.g., sharks) to feed on their blood and tissue using a suction mouth covered in teeth.

Jawed Fishes Classification

Jawed fishes can be divided into cartilaginous and bony fishes.

Cartilaginous Fishes:

Sharks and rays are examples of cartilaginous fishes.
They have skeletons made of cartilage instead of bone, making them more flexible.
Sharks must keep swimming to move water over their gills because their gills lack a covering.

Bony Fishes:

Bony fishes possess an operculum, a muscular flap that helps force water over their gills.
They have skeletons made of calcium, providing a rigid structure.
Bony fish are the first to have a skeleton made up of calcium.

Lateral Line:

Both cartilaginous and bony fishes have a lateral line.
The lateral line is a sensory organ that detects electrical impulses and movements in the water.
Sharks have highly sensitive lateral lines that can detect electrical impulses from animal nerves.
Helps them orient in 3D space.

Ray-Finned vs. Lobe-Finned Fishes

Ray-Finned Fishes:

Ray-finned fishes are typical fish with flexible fins.
Their fins are not suitable for use on land.

Lobe-Finned Fishes:

Lobe-finned fishes have fins supported by bony lobes, similar to a wrist and palm.
Lungfish are lobe-finned fishes known for having both lungs and gills.

Lungfish Adaptation

Lungfish can survive in areas with wet and dry seasons by burying themselves in mud and breathing through their lungs during the dry season.

Coelacanth

The coelacanth is another type of lobe-finned fish that was initially discovered as a 60-million-year-old fossil.
A living coelacanth was discovered in the 1960s, showing that the species had not gone extinct.

Transition to Tetrapods

Lobe-finned fishes eventually evolved into tetrapods (four-footed animals).
Tiktaalik is a transitional fossil that shows the evolution from lobe-finned fishes to amphibians.

Amphibians

Amphibians include frogs, toads, salamanders, and caecilians.
They live in water as eggs and juveniles before becoming adults that can live on land.
The term "amphibian" means living a double life, referring to their ability to live on both land and in water.
Amphibians require water for reproduction because their eggs are not designed to survive without it.
Amphibian species are more common in areas with more water and warmer temperatures.

Metamorphosis

Amphibians undergo metamorphosis, transforming from a fish-like tadpole into an adult form with legs and without a tail.

Reptiles

Reptiles developed the amniotic egg to protect their young from drying out.
Amniotic eggs are full of fluid and surrounded by a waterproof shell, providing nourishment and protection for the embryo.
Reptiles also developed scaled, waterproof skin to conserve water.
Reptiles include lizards, snakes, turtles, and alligators (referred to as non-bird reptiles).
Most reptiles lay their eggs on land.

Ectotherms

Reptiles are ectotherms, meaning they rely on external sources of heat (e.g., the sun) to regulate their body temperature.
Ectothermy has advantages, such as lower caloric needs, and disadvantages, such as difficulty living in colder areas.
Parenting levels vary among reptiles; some, like turtles, leave their eggs, while others, like alligators, guard their nests.

Birds

Birds are descended from dinosaurs (which were reptiles) but have undergone many adaptations for flight.
Adaptations for flight include:
- Honeycombed (not truly hollow) bones to reduce weight.
- Single instances of organs to reduce weight (e.g., one gonad, one kidney).
- Beaks instead of heavy teeth.
- Elongated sternum (breastbone) for muscle attachment.
Birds are endotherms, meaning they can generate their own heat, which requires them to eat more.
Birds have diversified into many different forms with specialized beaks, feet, and sizes.

Mammals

Mammals first appeared about 200 million years ago.
Early mammals were small, nocturnal burrowers.
Mammals became dominant after the extinction of the dinosaurs about 65-66 million years ago.
Key features of mammals are body hair and the ability to produce milk (lactate).
Mammals are amniotes, providing a protected environment for their young to develop.

Monotremes

Monotremes are mammals that lay eggs (echidna and platypus).
They lack nipples and instead sweat milk onto their fur for their young to lick up.

Marsupials

Marsupials have a pouch and a placenta.
They give birth to underdeveloped embryos that crawl into the pouch and continue to develop.
Most marsupials live in Australia (kangaroos, wallabies, koalas, opossums, wombats, Tasmanian devils).
The opossum is the only marsupial found in North America.

Eutherians (Placental Mammals)

Eutherians include most mammals (humans, cats, dogs, rabbits, elephants).
The placenta provides nutrients to the developing embryo, similar to the yolk of an egg.
The placenta allows for longer pregnancies and more developed infants at birth.
This leads to longer lasting associations between mother and young, allowing for more teaching and social bonding.