[Robertt (OP)]

I am in a discussion with a theist and he gave me a very weird question indeed.. one that I find a bit moronic and yet a bit interesting.. I then thought about it and I really don't know.. I am not a scientist but am interested in this area.. so if anybody has the slightest ides about this question.. much appreciated.

Which evolved first, the human skin, blood, or the human heart?

To me is seems like they all evolved together.. simple answer but from my current knowledge that seems correct.

[Bored chemist]

It probably depends on your definition(s)
If we take the view that "being human" means having some heritable characteristic(s) then it seems that humans are defined by having some particular bit of, or set of bits of, DNA.
There must have been a first individual with those characteristics (whatever they might be), and they were the first human.
That individual (probably) inherited those traits (some from their mother  and some from their father).

In that case they had that DNA as soon as they were a fertilised egg- and before they had skin or blood.
So the first human came first, before any of those bits- skin heart or blood.
So you need to look at the embryology of humans and see what bits form first.
As far as I can tell, it's the skin
https://en.wikipedia.org/wiki/Ectoderm
but it's a matter of interpretation

On a related note, the chicken was there before the egg formed round it.

[Villi]

My first thought was about the definitions and specificity like Bored Chemist pointed out.

You probably left out the human in blood, so I'll assume human blood and not just generic blood (whatever that is). I'm interpreting this question as what is most unique and most different from the last common ancestor of humans (homo sapiens), a great ape probably, to humans. Hearts are quite universal, transplant of hearts can occur between human and pigs, I've read, and probably apes. Blood is also somewhat universal and there are some differences in antigens and blood groups between species, but not enough that can't be edited such that transfusions are impossible. So the heart and blood is not uniquely human, but skin is probably what makes humans human more than anything. It was probably the first thing to evolve and distinguish the first member from its previous species.

More than anything, I would say the brain though.

[evan_au]

Which evolved first, the human skin, blood, or the human heart?

I would say the human DNA, which has the instructions to construct the human skin, blood and heart.

There must have been a first individual with those characteristics (whatever they might be)

I suggest that a precise definition of those characteristics is effectively impossible.
Evolution occurs within a population of individuals, each with different DNA.
All of the genes that make us human may have appeared in this population, without all of them being in any particular individual.

Today, with over 7 billion individuals, there is a considerable range of variation.

Evidence of interbreeding between non-African humans and Neanderthals or Denisovans means that these other humanoid species had genes which we now consider a normal part of the human genome (at least in some parts of the world).

I am in a discussion with a theist

Perhaps suggest that if humanity survives the next century (or millennium), and especially if humans manage to survive as a permanent population away from Earth (eg on Mars or living in space), it is not clear what we will become.

Our current self-destructive tendencies seems ill-suited to life on Earth.

Our current frailties seem ill-suited to a life away from Earth with its continuous 1g acceleration, 1 bar atmospheric pressure and protective atmosphere and magnetosphere.

Looking back, when would they think the first true humans appeared? Have they appeared yet? 

[Bored chemist]

...
Today, with over 7 billion individuals, there is a considerable range of variation.

...

Indeed, but back then, there was just 1.

[Jphynesuk]

Ignore all previous answers citing DNA.  Skin is found on archaea/Neomura which predates the heart or blood.
Blood is found in bacteria which predate the heart. The heart is only necessary in lifeforms larger than 7mm.

[Bored chemist]

Blood is found in bacteria

Really? what pumps it?

[evan_au]

Blood is found in bacteria

Do you mean that Cytoplasm is found in bacteria?
- Cytoplasm is the liquid in which a bacterium's organelles reside. But Cytoplasm doesn't contain red cells (containing hemoglobin) or white cells (for fighting bacteria).
See: https://en.wikipedia.org/wiki/Cytoplasm

Or did you mean to say that "bacteria are found in the blood"? This is a serious condition called septicemia.
See: https://www.healthline.com/health/septicemia