Note: The logical and metaphorical structure of this writing is taken from "The Human Basis of Laws and Ethics: Without God, How Can You be Moral?" By Frederick Edwords. The words, however, are my own.
The argument for the humanistic construction of morals must begin with nothing. In nothing, there is nothing to bitch about (and of course no one to do the bitching) so there are no morals.
Now, let us add some beings to the mix. These beings are not human. They have no needs and no desires. They simply exist, and just keep on existing, observing the world around them and not interacting with it. They have no needs or desires, and therefore no conflicts. Without conflicts, there is no need for morals, and therefore there are no morals, given the lack of any drive to create them.
Now let us add a Human to the mix. For the sake of simplicity, let us call this human Adam. Adam has arrived in the world with a full set of wants, needs, and desires. He can experience both pain and pleasure, and usually strives to avoid the former and seek out the latter. Adam is now faced with decisions. He begins to question everything around him, asking "is this for me, or against me?" Only here do morals begin to emerge, for Adam must decide, in order to survive, that things which are apparently for him should be sought after, while things that are against him must be avoided.
This, however, does not complete the evolution of morals, for Adam is a sole being representing the whole of the human race when, in fact, humanity is made up of many beings. So let us introduce another human to the scene.
Enter Eve. Eve has most of the same needs as Adam, and many of the same wants and desires. However, there are instances in which their desires come into conflict with eachother. So Adam and Eve must invent a system by which the greatest possible good can be achieved.
As it turns out, Adam and Eve kinda like eachother. They start to feel that their needs (and ever their wants and desires) can be much better served if they each work towards the good of the two of them together, rather than competing for what they want as individuals. So a system of morality comes to life as the rules for interactions between individuals, established by the individuals they efect.
But what about God???? Let us see what happens when we add God to the mix.
One day, Adam and Eve happen to stumble upon a situation in which their wants have come into such a conflict that there is simply no resolution in sight. Finally, a lightbulb appears over Adam's head, and he exclaims, "but aren't we forgetting about God?"
"What in the world are you talking about?" Replies Eve, quite perplexed.
"Oh...You know God. He's the one that put us here. The one that lays down what's right and what's wrong!"
To which Eve replies, "Oh. Well, I'm not sure how we got here, but I am pretty certain that all these ideas
we have taken to living by about the right way to treat eachother came from us."
"But that's where you're wrong, Eve. God made up all those ideas, but he calls them 'rules.' We have to live by these rules, because God made them. And guess how God feels about this particular argument that we're having today?"
"How's that, Adam?"
"He Agrees with ME!"
So, clearly, morality can exist perfecly well on its own. God just complicates matters.