Something well designed is as simple as it's possible to be whilst still performing the function it was designed to perform. It would be possible to make spear points out of something other than flint - metals, bone, even wood - but you would be hard-pressed to come up with a better shape than those made by neolithic people. The humble garden dibber is hard to better for design. You can take a decent knife or hand-axe to any hedge and probably find a piece of wood which you can make a decent dibber from in a few minutes.
Both the spear point and the dibber are perfectly designed for a purpose and the purpose is obvious. There are no moving parts and minimal maintenance needed.
And this, of course, is how we can, at a glance, tell they were intelligently designed. Their lack of unnecessary complexity gives that away.
Compare that to the design of the human body (or any other living organism, for that matter). The human body is immensely complex compared to a spear point or a dibber.
Starting at the cell level, where probably the most complexity is to be found:
So, if humans are designed, as creationists insist, what purpose justifies this huge level of complexity, very much of it apparently redundant at that, especially when we look at the genome.
Let's see what the Bible says about the purpose of humans:
And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
But we can dismiss that because God contradicts himself in Genesis 2.
And the Lord God said, It is not good that the man should be alone; I will make him an help meet for him. And out of the ground the Lord God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof.
Besides, simply being here to look after the animals that were put here to help us... er... look after them, seems a bit pointless somehow. One might have expected an omniscient, omnipotent designer to have thought up something a little more challenging. Why couldn't it have have made them able to look after or 'have dominion over' themselves, for example? But then, they wouldn't have had any purpose either...
And what of the species we're still discovering, and those living at the bottom of deep oceans? They seem to have been okay without our 'dominion', don't they?
So where else can we find our 'purpose' according to Christians?
Let us hear the conclusion of the whole matter: Fear God, and keep his commandments: for this is the whole duty of man.
And that appears to be about it. Everything else seems to be about being 'saved', which is not much of a purpose really.
God created us so we could be saved from him? Is that it?
But, whatever this purpose is, why did it need this huge complexity? Surely it should not have been beyond the wit or abilities of an omniscience, omnipotent god to make something simple which could fear him and obey his commands. Why did it even need to be multi-cellular? Why, even as a single-celled organism, did it need the fantastic complexity of the eukaryote cell with its vast genome needed to produce all those enzymes for enormously complicated biochemical pathways, respiration, cell reproduction, and so on.
Surely, this purpose could have been achieved with the simplest of simple entities if the creator had the powers and abilities Christians attribute to him? But even if size was important to God, why couldn't he have made bodies which are much less complex? Maybe ones which don't consume energy so have no need to eat and metabolise. How about ones which just bud off new individuals, so all this sexual reproduction and the means to find and mate with a partner, then spend years rearing the children was unnecessary? How about telepathy so we can communicate simply and don't need speech, reading, hearing, etc?
In short, why does it look like we have a purpose which is unconnected with what Christians claim their god had in mind?
You see, the problem is, the vastly unnecessary complexity for such a nebulous purpose is not evidence of design, especially of intelligent design; it is evidence of unintelligent, undirected and purposeless design, just as one would expect of a mindless, purposeless design process like evolution by natural selection where the only function is to produce individual gene vehicles in order to produce the next generation of gene vehicles.
I'm afraid, whoever told you that complexity is a problem for evolutionary biology has mislead you. It is a problem for creationism and most of all for its under-cover wing, the 'intelligent design' industry. Biological complexity is exactly what we would expect if the 'creator' was an unintelligent, mindless, purposeless process. An intelligent perfect designer would have created a perfectly simple design.
It really is time religious people reassessed their superstitions and realised just how silly they are. As a basis for giving life a genuine meaning and purpose, they are about as useful as a back pocket in a vest, or a chocolate teapot.