You have some great independent thinking going on
here. It may help to remember that people like to
put things in categories because it makes us more
efficient, but the natural world does not always
fit neatly into our categories.
One category is the Kingdom Animalia. Euglena
are not animals. They do move and eat, which are
animal characteristics, but animals have to be
made of multiple cells. Euglena belong to the
Kingdom Protista. You are right that plants do
photosynthesis, but so do other things, like
Euglena have evolved over time, and may
continue to change in the future, but no one can
predict what the changes will be. It all depends
on random mutations and how useful those mutations
are in the changing environment. For example,
let’s say that some Euglena were trapped in a
cave. They couldn’t do photosynthesis without
light. Chloroplasts and chlorophyll are expensive
to make. So if a Euglena happened to have a
mutation that caused it to make fewer
chloroplasts, or none at all, it would probably do
a lot better than the other Euglena. It could
invest in other things, divide faster, and leave
more copies of itself. Eventually, the entire
population of cave Euglena might be without
Being in the dark would not cause the mutation.
Mutations are random. It’s just that being
chloroplast-free would be an advantage in a cave.
A mutation to be chloroplast-free in a place with
light would be a disadvantage, so those Euglena
would divide less than their competitors.
I don’t think they are likely to be very
intelligent since they only have one cell, but I
guess it depends on how you define intelligence.
If you just mean that they can move and respond to
their environment more than plants can, then they
are already there.
What do you think intelligence means? When a
plant faces the sun, is that behavior?
Thanks for asking,