For quick rebuttal to the notion of a designing intelligence being the only logicl explanation for the complexity of biology, I give you The Designer's Goofs, all 96 of them (bonus points for finding more). The most persuasive IMO are those where the supposed designer made the same exact mistake in many species. Rather odd sense of humor he has.
Primate dietary requirement for vitamin C
Apes and humans require vitamin C in their diets... which is rather odd, because most mammals synthesise their own. Yet although we humans cannot; we do have the same gene for this that they do... but it is broken! And it is rendered non-functional by precisely the same mutation in all the great apes. Coincidence? And how loving of the creator to give people without adequate diets scurvy!
Unlike most mammals, cats are uninterested in, and presumably are unable to taste, substances that are sweet. 'So what', you might say, 'they don't need to'. Well perhaps. But if they don't need to taste sweet things, it is odd that they possess the same requisite genetic machinery for sweet detection that other mammals have... but one of the two receptor genes is broken, rendering it non-functional.
What's more, the exact same deletion and stop codons are found, not just in domestic cats, but also in tigers and cheetahs, which means the designer gave superfluous sweet-taste genes, and then broke them identically, not just to one design, but to several.