1. The Earth - a perfectly sized, perfectly balanced ecosystem to support human life. Consider the temperature swings we encounter, roughly -30 degrees to +120 degrees. If the Earth were any further away from the sun, we would all freeze. Any closer and we would burn up. And our moon is the perfect size and distance from the Earth for its gravitational pull. The moon creates important ocean tides and movement so ocean waters do not stagnate, and yet our massive oceans are restrained from spilling over across the continents.
2. Water - a perfect life sustaining liquid. Perfect boiling and freezing points. Chemically neutral, allowing food, nutrients and minerals and medicine to be perfectly absorbed into the body. 97% of the Earth is oceans, but there is a perfectly balanced mechanism to separate the water from salt and disperse it to enable plant life and human life to thrive.
3. The Human Brain - a perfect balance of auto and controlled responses. Processes more than a million messages a second filtering out the noise and nonsense (these forums) automatically allowing focus on the world around it.
4. The Human Eye - can distinguish among seven million colors. It has automatic focusing and handles an astounding 1.5 million messages simultaneously.
The odds of human life on Earth beginning as an accident or via evolution are astronomical to the point of being almost null. Forming a single DNA strand is extremely more complex than lining up a few DNA strands. And DNA itself is not life. It must exist within a living cell that has ribosomes, plasmids, cytoplasm and all sorts of other stuff. To expect all of this to have occurred on our humble planet within a mere few hundred million years requires a tremendous leap of faith most people (with undamaged brains) would not be prepared to make.
But don't take my word for it. Here's Professor Francis Crick, awarded the Nobel Prize for the discovery of DNA:
An honest man, armed with all the knowledge available to us now, could only state that in some sense, the origin of life appears at the moment to be almost a miracle, so many are the conditions which would have had to have been satisfied to get it going.
The trouble is that there are about two thousand enzymes, and the chance of obtaining them all in a random trial is only one part in (1020)2,000=1040,000, an outrageously small probability that could not be faced even if the whole universe consisted of organic soup. In terms of complexity, an individual cell is nothing when compared with a system like the mammalian brain. The human brain consists of about ten thousand million nerve cells. Each nerve cell puts out between ten thousand and one hundred thousand connecting fibers by which it makes contact with other nerve cells in the brain. Altogether the total number of connections in the human brain approaches 1015 or a thousand million million. Numbers in the order of 1015 are of course completely beyond comprehension. Imagine an area about half the size of the USA (one million square miles) covered in a forest of trees containing ten thousand trees per square mile. If each tree contained one hundred thousand leaves the total number of leaves in the forest would be 1015, equivalent to the number of connections in the human brain! Despite the enormity of the number of connections, the ramifying forest of fibers is not a chaotic random tangle but a highly organized network in which a high proportion of the fibers are unique adaptive communication channels following their own specially ordained pathway through the brain. Even if only one hundredth of the connections in the brain were specifically organized, this would still represent a system containing a much greater number of specific connections than in the entire communications network on Earth.