Most of this is simply from history and the texts left by various peoples,
from which it can be shown that about 1000 years is enough to split
languages. And yes, the splits almost always involve the separation of
various peoples into different political groups which produces less
commerce and social interchange. The process is entirely analogous with
evolution. The only thing different is words replace genes and the rate at
which the process occurrs. If you assume a 25 year generation, then if you
line up you and your 40 nearest ancestors, each person in the line can
communicate with the person immediate next to themselves, but the two end
people probably can only smile and nod at each other.
Here are some citations to back up the 1000 year time frame:
"The procedure followed by the gloottochronologists, when the time of
splitting of two related languages is to be deterined, is to list the
equivalent words fromt he languages underconsideration, and not the pairs
which, on the grounds of their similarity (taking into account the known
laws of sound shift) appear to be or are known to be cognates. These are
then assumed to be retained from the common ancestor language, while the
words which have different forms in the two languages under consideration
are assumed to differe because the original word has been lost in one or
other (or indeed both) of the languages. THe number of word-pairs which
are cognate, out of the original ist of a hundred, is thus a measure of the
closeness (of the retention of the basic core vocabulary) of the two
languages. Conversely the number of pairs that are now different are an
indication of the extent to which words have been lost, and hence a measure
of the time since the two languages originally separated.
"It the original study, various pairs of languages were compared such as
Old English and Middle English, the Latin of Plautus and early modern
French, ancient Chinese and modern Mandarin and so on. It was concluded
that the average retention rate was 81 percent per millennium. When the
original word list was shortened to the hundred words listed above, the
rate was adjusted to 86 per cent per millennium.
"Various computations have been undertaken on this basis, offering as
conclusions that Spanish and Portuguese split at about AD 1586, Italian and
French at the same date, Romanian and Italian in 1130 etc. English and
Dutch would have split in AD 860 and English and German in 590 and so on.
"Some linguists have criticized these calculations on the grounds that they
do not always give precisely the right answers. It is known, for instance,
that the Germanic languages became separated at rather earlier dates than
those proposed. But to my mind the astonishing thing is that the answers
from such calculations are in some cases so close to a date of
differentiation which can be established on independent grounds." ~ Colin
Renfrew, Archaeology & Language (New York: Cambridge University Press,
1987), p. 115-116
"In the 3000 or more years it took for Bantu-language speakers to migrate
from Cameroon/Nigeria to Central and South Africa, there was ample time for
Bantu languages to diverge, but their original unity is easily appreciated.
In general, two languages differentiate to the point that mutual
intelligibility is lost in 1000 years (order of magnitude) and often less.
" ~ L. Luca Cavalli-Sforza, Paoli Menozzi and Alberto Piazzi, The History
and Geography of Human Genes, (Princeton: Princeton University Press,
1994), p. 102
glenn
Foundation, Fall and Flood
Adam, Apes and Anthropology
http://www.flash.net/~mortongr/dmd.htm
Lots of information on creation/evolution