AMPHIBIAN EVOLUTION

According to the fossil record, here is approximately when the major groups of land animals appeared:

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Life originated in the seas. The first animals were simple ones without backbones -- "invertebrate" animals such as segmented worms, sponges, and corals. The first fish were so simple that they didn't even have jawbones for opening and closing their mouths. The first land animals with backbones were the amphibians. The oldest amphibian fossils look like fish with thick, muscular fins. That's because amphibians arose from early fish. The first amphibians lived more in water than on dry land.

So:

• amphibians arose from fish
• reptiles arose from amphibians
• birds arose from reptiles
• mammals also arose form reptiles

Keeping the above points in mind, we can say that, from an evolutionary viewpoint,   amphibians as a class developed before many of nature's most useful animal "inventions" had appeared. For instance, look at what amphibians don't have (and this includes today's amphibians):

WHAT THEY CAN'T DO

• They can't mate on dry land. Like their fish ancestors, amphibians must return to water to reproduce. Female amphibians lay gelatinous eggs in water, and then the male deposits clouds of sperm over them. This is one reason why amphibians are seldom found far from water, and one reason they may be rare or nonexistent in your backyard. Check out our Frog Reproduction page.

• They can't retain water well in their bodies. Scales, which cut down drastically on water loss from skin, didn't appear until reptiles arose millions of years after the first amphibians. This is another reason why amphibians need to stay near water.

• They can't keep their bodies warm during cold weather. Amphibians are "cold blooded." When the air cools, so does their body temperature. This causes the animal to become sluggish, which can be bad if a predator is near. We think of reptiles as being cold-blooded, but some evidence suggests that certain dinosaurs were "warm blooded," like the reptiles' descendents, the birds and mammals.

THE WATER PROBLEM

In most backyards, toads are the most likely of all amphibians to appear. That's because toads have a special adaptation that helps them range farther from water than other amphibians: Their thick, warty skin keeps water inside the toads better than other amphibian skins.

Because water and air so easily flows into and out of amphibian skin, amphibians are much more vulnerable to pollution in water and air than other higher animals. In fact, today amphibian numbers are collapsing all over the world. Acid rain is thought to be responsible for amphibians disappearing from streams, lakes and ponds where they were common only a few years ago. Nowadays we often hear of two-headed or five-legged frogs. Apparently this high frequency of deformity results from pollutants interfering with natural development at the genetic level. Of course, as pollution levels increase, eventually other kinds of animals, even humans, are sure to suffer similar fates...

You can review some amphibian-oriented books available at Amazon.com in both the US and the UK by clicking here.