The word 'metamorphosis' means 'change'; it's an English word taken from the Greek word μεταμόρφωσις, which literally means a change of shape. Metamorphosis is a key element of biological evolution, seen clearly in the change of animal species from one to another, sometimes with dramatic changes in shape.
In a similar way to evolution, all human languages change over time. Just like evolution, this process eventually produces new languages, and results in the death of other languages. Some languages change faster than other, depending on the environment and the various pressures which cause language developments.
In many ways, the change of language is like evolution. However, in some ways it is quite different. Over time, evolution has produced a massive number of different species on our planet, which (as a general trend), started from very simple life forms which developed into more and more complex organisms.
However, the development of languages over time is very different. Although it is natural for us to think that earlier languages from less modern societies must be very simple, and that languages have become more complex over time as our societies have become more sophisticated, the opposite is true; older languages are typically much more complicated than modern languages.
What we find, interestingly, is that over time languages show a general trend of becoming simpler. This kind of change involves a combination of different influences which may not all apply in each situation, but some of them are typical to any language change. These influences will be described in the second post of this series.
 'All human speech varieties are always in a constant process of slow transformation into what eventually will be so different as to be a new language entirely. This change is certainly influenced by historical, social, and cultural conditions but is not caused by them alone; the change would continue apace even without these things.
Just as we can understand biology only by being fully aware of the centrality of evolution to how life as we know it arose and will develop, we can truly understand language only by shedding the Monopoly-instructions conception that school inculcates us with and replacing it with a conception of language as a fundamentally mutative phenomenon.', Whorter, 'The Power of Babel', p. 16 (2003).
 'One might quite reasonably suppose that a First World culture with tall buildings, cappucino, and Pokemon would have a grammatically "richer" language, necessary to convey the particular complexities inherent to our treadmill to oblivion, whereas preliterate cultures such as, say, those in the Amazon rain forest would have "simpler" languages for simpler lives. "Bunga bunga bunga!!!!" as the natives say in old cartoons.
Ironically, however, if there is any difference along these lines, it is the opposite: the more remote and "primitive" the culture, the more likely the language is to be bristling with constructions and declensions and exceptions and bizarre sounds that leave an English speaker wondering how anyone could actually speak the language without running the risk of a stroke. Meanwhile, many of the hotshot "airport" languages are rather simple in many ways in comparison with the "National Geographic" cultures' languages: English, Spanish, and Japanese grammar are "Romper Room" compared with any language spoken by the hunter-gatherers who first inhabited the Americas.', Whorter, 'The Power of Babel', p. 6 (2003).