Was the 51% attack a real threat in the early days of bitcoin existence? The hypothesis of 51% attack is based on a malicious entity is able to control more than 50% hashrate of all mining nodes. It's understandable that there's huge amount of mining power existing nowadays, however, they grew up gradually since 2009.

I'm wondering when there were only a few of people who knew the bitcoin back to the 2009, therefore, surpassing 50% of mining nodes was fairly easy(for instance, assuming there were only 10 participants in the early days of bitcoin). Because controlling more than 50% of mining power probably just needed to control more than 10 computers in the earliest days since bitcoin birth.

As a result, do I need to worry about the 51% attack possibly happened in the blocks of which the height number is small and are those blocks trustable?
Yes it was easier back then, that is why ASICs are a good thing and keep the network much safer. But no you don't need to worry about it, even a 51% attack can only do a limited number of things to the network, blocks and transactions must still be valid. So if a 51% attack had occurred, for example double spending a transaction, that is long in the past and would definitely not affect you now.