There are two parts to the answer on this one. The first is that linguistic drift is a natural phenomenon. Prior to rapid travel, instant communication, and wide-spread literacy, two villages could develop different accents or dialects that were mutually incomprehensible even if they were only 30 miles apart. We see evidence of this in studies of accents in England from the Middle Ages. The drift occurs in the pronunciation of both vowels and consonants, in contractions, and, more slowly, in grammar and syntax. So it is quite possible that, over 50,000 years, 7,000 languages could have formed from one original as people migrated around the entire planet and lost touch with one another. (It would be very surprising if, after 20,000 years apart, a 'Kung Bushman and an Inuit Eskimo could understand one another!)
The second possibility is that, when humanity spread out from the original tribe of Eve (either evolutionary or Biblical), the capacity for language existed, but language was not fully developed. So a few core languages might have developed completely independently. This might account for some core structural components of language. That is, things like some languages (like Chinese) being tonal, while other are atonal, some being syllabic (like Japanese) while others are alphabetic, some using clicks, like 'Kung, while others do not: Factors such as these might arise from very early independent evolution of complex languages.
Two facts remain: Most changes in language are simplifications. Older languages in all language groups tend to be more complicated. (For example, English used to have singular, dual, and plural forms. Now it has only singular and plural.) And, even with the advent of modern communications slowing the change down, languages are still changing.