evolution

I wonder until which a time the languages have had the same evolution. I
mean, there must be some proceses which happened in all IE languages,
and after a time, proceses which appear just in one language.
For instance, the change of "d" to "z" in some languages. This was a
process which is supposed to have evolued in every language after they
became separated.
But sometimes the actual way how they look likes, show very, very big
similarity.
I wil like to take as example 2 verbs from two very different IE
languages. I will take german and romanian.
The one is a germanic one, the another is a "romance" one.The verbs are
" to be" and " to say". I will put here the conjugation for present just
for seeing how they look like.

Infinitive form for to be: ger="sein", rom="a fi" ( from a supposed
auxiliar latin *fire)
for english I am, you are, s/he is, we are,you are,they are we have:

ger: bin, bist, ist, sind, seid,sind
rom: sânt,esti,este,sîntem,sînteti,sînt

1) I write in rom. example with "the old" orthography ("î" instead of
actually "â") for a better comparation.
2) If we make abstraction of pers. I and II sg we see that the
conjugation is very similar and they have both the
cluster "nt/nd"

For the verb " to say" the similarity is more interesting.Please keep in
mind that the german "s" is pronounced like rom. "z" (as the english "s"
in "case"

infinitive form for " to say": ger=zagen, rom=zice ( from latin dicere,
pronounced dikere)

ger: sage,sagst,sagt,sagen,sagt,sagen
rom: zic,zici,zice,zicem,ziceti,zic

Of course, just for these similarities none will think that rom. lang
and germ.lang are in a parental or familial relationship. They are in
the same family of IE and that is all.
The question here is : how does it happen in two languages, different as
family, we find the same fonological stamps, specialy when one is a
"romance" one and the another a germanic one ( with 2
Lautverschiebungen).
Please do not think just at a simple coincidence. There are more
exemples here; I limited myslef to give just 2 but if someone want more,
I can give more.

The question is: are here separate developments or there are
developements which happened in every language at a different time? If
we think that rom.lang= latin lang., that means, the change in romanian
of latin "d" in "z" was somewhere after IV centuries e.n. But in german?
So far I know, the first Lautverschiebung is not exactly to determinate
in the time line. The second one is somewhere between V-VIII
centuries.But the second Lautverschiebung is not interesting for me in
this case because then the germanic changed the "th" to "d" or "t" and
2t" to "z" ( german "z" like in Zeppelin)
In the examples I gave interesting is the "th" which was after the first
Lautverschiebung.And that means until IV centuries so far I know.
From the gothic, we know they used already "th" in the gothic glosses we
have. In this case I ask myself:
-german influence in romanian "zice"?
-separate evolutions in every language?
-phoenomen which happened in common somehow?

best regards

Alex