Many Christian apologists and creationists love to use the so-called "watchmaker argument" to defend the idea that a creator god must exist and is the only logical explanation for why life, including us, exists.
In its simplest form it goes something like this: Suppose you are walking down a beach and encounter a watch. You examine and find out that it works, and about its complex internal details and mechanisms. It would be absolutely ludicrous to think that it came to be on its own, due to random natural phenomena only. Quite clearly someone created it. A watch requires by necessity a watchmaker. The same is true for life: Living organisms are way too complex for it to have happened by random chance, and thus there must have been an intelligent creator.
There are countless explanations and refutations to this that point out how biological organisms are quite different from artificial mechanisms like a watch, and how natural processes can perfectly well explain the formation and diversification of biological organisms.
Here, however, I'm going to approach that argument from the opposite side, the side that's very rarely if ever discussed. In other words, I'm going to tackle the claim that if we find a complex mechanical construct, it must have been created by an intelligent being.
Suppose that one day in the future we are able to traverse huge distances and visit other planets, and one day we find a planet that's populated by billions of robots of all sorts. These robots might look like spiders, crustaceans, perhaps even some kind of theropods. And they are fully mechanical, made of metals, tubes, intricate clockwork mechanisms etc, as well as some kind of strange electro-mechanical contraption that serves as the "brain" of the robot. Crucially, this is the only kind of "life" that exists on the planet; there is no biological life of any sort.
Quite naturally the first logical conclusion is that some kind of intelligent alien race created these robots and put them on that planet, maybe as some kind of experiment, or some kind of "robot zoo", or the like. This is the most logical conclusion because it conforms to our own experience about such mechanical constructs.
However, suppose that we keep observing this planet and its billions of robots, and notice that they actually have a limited lifetime, and they reproduce, and their offspring is always slightly different from its parents, rather than being identical copies.
This would be really interesting, and would already hint at something more complex going on, although the hypothesis that they were created and put there by some intelligent alien species would still be by far the most logical conclusion. Maybe the alien species made the robots like that.
However, suppose we start digging the ground, and we start finding dead robots. Fossils of sorts. And we keep digging more and more, and dating the different layers of sediment, and we observe that the deeper we go, and thus the older the robots we find are, the more different they are from the current alive ones. We go so deep that this geologic column goes back millions, tens of millions, even hundreds of millions of years. It turns out that these robots have existed for that long on this planet.
Moreover, as we dig deeper and deeper, going back in time hundreds of millions of years, the design of these robots become simpler and simpler, but we see a logical progression over the millions of years of how the current alive robots came to be from those simpler robots from hundreds of millions of year ago.
It might even be that when we go back enough in time in the geologic column, we find evidence of more organic compounds having been part of these ancient robots. Organic parts that have since disappeared in more recent robot "fossils".
Now this changes things quite a lot. This is strong indication that the current robots roaming the surface of the planet most likely were not directly created nor put there by some intelligent species. Instead, this is quite clear indication that the current robots literally evolved and diversified over hundreds of millions of years from more primitive forms to their current ones.
This is, in fact, what we observe in actual reality in our planet: We have strong evidence that current lifeforms were not somehow popped into existence as they are currently, but evolved over hundreds of millions of years to their current forms from more primitive life.
Of course this still leaves open the question of how the first ever robots on that hypothetical planet, or the first ever life in our real planet, came to be. However, the above alone is an argument against the "watchmaker" argument, as it's not completely impossible for complex mechanical devices to form without the intervention of an intelligent "creator".