Most of us like the smell and the taste of fried or grilled meat (potatoes
...hmmh). Obviously this preference has something to do with substances,
that are produced during the Maillard reaction. But chemical analyses do
not explain why we like these substances. Are we only imprinted to mothers
steaks or does this reflect an adaptational process, fixed in our genes?
Have our nutritional preferences changed to cooked and fried foods as an
adaptation to a long hominid use of fire?

I do not think so. Many animals like fried food too (just a personal
observation; sorry, no references....). The reason could be the production
of opiate-like substances during the browning process. This hypothesis is
supported by the findings of a new study: Exploring the Maillard reaction,
Wang et al. detected two novel substances belonging to the class of
DF-Carbolines. DF-Carbolines are psychoactive alkaloids and can be found in
tobacco, in the hallucinogenic beverage ayauasca and some are produced in
the organism after consumption of alcohol.

Reference: Mingfu Wang, Yi Jin, Jiangang Li, & Chi-Tang Ho (1999). Two
novel DF-Carboline compounds from the Maillard reaction between xylose and
tryptophan. Journal of Agricultural and Food Chemistry 47:48-50