True enough, humans are very evolved animals with an extremely complex mind. What was meant is that there is nothing beyond the physical basis of the mind, at least for some agnostics/atheists.
I, myself, don't feel the need to add in a soul when the human brain his sufficently complex to generate things like conscience, emotions, logic and so on and so forth.
As for the energy released when some body dies, well there is none. The human machine is no diffent then a car. Crash a car, it is broken an can no longer extract energy from its fuel to transform it into kinetic energy.
An animal is the same, you just go from a state of order towards chaos, increasing entropy. The only difference between the car and us, is that we rot way faster than the car rusts.
What is a more interesting question is how we've evolved brains that feel like somebody is ''taken away'' when we see them die. This is probably where some notions of souls emerged. We just look ''different'' when we are dead.
Anyway, for me religion kinda lost its meaning when you ask questions in 5th grade and the teacher can't answer, or when you just can't imagine a God like the one written in various Holy Books, or when you actually start learning about various religions and you realize that most stories in the Holy Books of Abrahamic religions are mostly taken from preexisting cultures, or have had additions according to the prevailing cultures/religions of the times.
Beyond that, it feels a bit weird when you read the holy books of the main monotheist religions and read about them and realize you could have done a better job yourself at creating one.
However, some more eastern philosophies are more in synch with what I believe humans should strive towards, the New Testament is also pretty good if you remove the religious stuff.
In the end, it would probably be easier if I could feel or ''know'' that there is a supreme being looking out for me, that has a plan for me and that's going to take me into his bosom upon death and give me everlasting joy and contentment...or send me to Hell for all eternity.