Lecture 32—Vertebrate Evolution—Adaptation to Land

In this lecture we continue to identify where key characteristics evolved in the vertebrates. When they invaded the land environment they faced the same problems that the plants had earlier. Actually, the arthropods were there first by 530MYA. All faced new problems of physical support, water balance, temperature fluctuations.

In this lecture we follow the development of limbs from fins; a shift from water breathing to air breathing (gillslungs); a shift from poikilothermic to homeothermic temperature regulation; the development reduced water loss by the skin and excretion via a water conserving kidney; a dramatic change in reproduction from a water to land with less dependency on water (eggs once laid in watereggs developing a waterproof shellinternal development of eggs in the maternal reproductive tract.

The basic limb structure was established in fish: paired pelvic and pectoral fins led to four-limbed animals. The basic bone structure first appears in lobe-finned fishes.

Respiration: Notice that gills (fleshy projections that flap around in water as they pickup O2 and get rid of CO2) are not effective in air (they collapse) and lose huge amounts of water. Lungs evolved out of the digestive tract as fish gulped air when in low O2 environments. In amphibians and early reptiles the lungs were simple sacs. In later reptiles, birds and mammals, they became elaborate chambers with tiny internal compartments. The huge surface area for greater exchange of gases with allowed them to have much higher metabolic rates and activity.

 Agnatha (jawless fishes)----Gills

 Fishes with jaws----Gills & lungs

 Amphibians----Gills (larvae); lungs (adults)

 Reptiles, birds, mammals----Lungs

Temperature Regulation: Invertebrates, fish, amphibians, and reptiles with few exceptions cannot control their body temperature. Their body temperature is directly dependent upon the environment; they are said to be temperature conformers or poikilotherms (=ectotherms). In order to survive they must either tolerate extreme temperatures or avoid them. Birds and mammals have independently evolved high metabolic rates producing large amounts of heat and conserving it by body insulation (feathers or fur). So they can survive in cold environments. Overheating is avoided by evaporative cooling (panting sweating, etc.). We say birds and mammals are temperature regulators or homeotherms (=endotherms). They can be active in a broad range of environmental temperatures.

Water Balance: A move to land requires that the animal reduce evaporation by developing an impermeable skin plus saving water by developing efficient lungs and kidneys. We see this shift when we look across the vertebrates:

 Fish----”No problem”

 Amphibians---- Big problem

 Moist skin & gills

 Toads decreased skin permeability

 Reptiles----Water loss

 Dry dead skin, scales

 Good kidney

 Internal lungs

 Birds & mammals----Water loss

 Dry dead skin & fur & feathers

 Very good kidney

 Internal Lungs

Reproduction: The first vertebrates likely laid their eggs in the water as sperm was deposited over them. Using fish as an example, zygote development was aided by a yolk food supply (analogous to the role that endosperm plays in seed development). Thus we see yolk sacs in all vertebrates including mammals but in the latter the yolk sac is vestigial. Fish and amphibians wrap their eggs and yolk sac in a gelatinous covering and laid in water.

Reptiles have evolved an egg designed to withstand dry conditions. The embryo is wrapped in three embryonic membranes: amnion = surrounding the embryo and yolk in a dilute salty solution; allantois = a vascular membrane extending toward the inside of the shell surface with numerous blood vessels leading back and forth to the embryo. The blood vessels carry O2 and CO2 back and forth from the embryo to the shell’s tiny pores for respiration; chorion = membrane enclosing the embryo, yolk, and other membranes. Surrounding everything is the leathery shell. The egg is self-contained in the sense that once it is laid nothing gets in or out except O2 and CO2 until the young reptile escape. It is truly an egg specialized for land; it is the critical adaptation for success on land. Not surprisingly, birds have the same type of egg except the egg shell is calcified.

Mammals also have the 3 embryonic membranes; thus reptiles, birds and mammals are all called amniotes. Reptiles, birds and the earliest mammals (=Monotremes like the platypus) surround the embryo and its membranes by a water tight shell. Marsupial and placental mammals do away with the shell and retain the embryo inside the reproductive tract. The yolk sac becomes vestigial as the embryo gains nutrients directly from the mother’s uterus by way of an extensive blood supply.

Terms/Concepts to Define

Amphioxus

Pikaia

Paired appendages

Agnatha

Ray-finned fishes

Lobe-finned fishes

Chondricthyes

Osteichthyes

Tetrapods

Air sacs

Alveoli

Poikilotherm (ectotherm)

Homeotherm (endotherm)

Amnion

Allantois

Chorion

Amniotes

Yolk

Placenta

Can you answer these questions?

  1. Why do we think that the lobe finned fishes were the ancestors of the tetrapods?
  2. What were the first land animals? What is the evidence?
  3. Identify the steps in the evolution of vertebrate legs.
  4. What is the basic bone structure of the foreleg of a vertebrate and where in phylogeny did it first appear?
  5. What are three major adaptations to land that we find present in both animals and plants?
  6. Arthropods were the first land animals. Based upon your knowledge of the adaptations that plants and the vertebrates made, predict what adaptations to land we might find among the arthropods.
  7. Why was there a shift in the mode of respiration in vertebrates as they left the aquatic environment?
  8. Reptiles and amphibians are largely absent in the arctic, suggest possible reasons for their absence.
  9. What are the common features of all vertebrates?
  10. Draw and label the parts of a chicken egg.
  11. Yolk is the food supply for the developing vertebrate embryo yet it becomes vestigial in the mammals. How could this have eveolved?