This is the 152nd most frequent Finnish word.
vuosina
"In the years."
Here, 'vuosina' is used to specify within which years (a plural period, 'early years of the 1970s') the changes occurred.
1970-luvun alkuvuosina tapahtui merkittäviä muutoksia.
In the early years of the 1970s, significant changes occurred.
'Vuosina' indicates a plural time period ('past years') within which the mentioned events have taken place.
Menneinä vuosina olemme todistaneet suuria teknologisia edistysaskeleita.
In past years, we have witnessed great technological advancements.
The word 'vuosina' again refers to a plural timeframe ('those years'), providing context and timeframe for the described situation.