A claim is extraordinary if it is inconsistent with our background knowledge. That’s what it means for a claim to be extraordinary. It is well outside of expectations, and expectations are based on our previous scientific collective experiences (not personal testimonies or personal incredulity).
That is, if something has never before been observed, and yet is claimed to have happened without leaving any evidence behind, then the claim is extraordinary.
We have no background knowledge that any intelligent designer (other than ourselves) is messing around with the evolution of living organisms, or that anything is required to mess around with it. But we do have observed all the mechanisms and processes that combine to yield what we know of as the process of evolution. Natural selection, mutations of all types, genetic drift, speciation, their interactions with environments etc. etc. And we have evidence that these processes were also occurring in the historical past.
Hence the extraordinary claim is that there is some mysterious intelligent designer that operated in the deep geological past to guide and steer life’s evolution. We just have no reason to think that is required, and it seems to have left no evidence behind.
First of all @Gilbert asked about “the passage from the first single-celled organisms to humans”, not the origin of life from non life. We just have no evidence that intelligent designers (other than ourselves) are intervening in life’s evolution, but we do have enormous amounts of evidence for observable natural processes operating just fine on their own.
Second, we also have no observation of anyone creating life from non-life, and the only intelligent designer we know of (ourselves) is a product of evolution, not it’s source. That again makes the invisible intelligent designer operating in ancient geological history the extraordinary claim.
Third, since life’s origins are actually unobserved, and we strictly don’t know what mechanisms or processes were involved, at best that would leave us in a position of having to say we don’t know how life originated.
Fourth however, there is actual evidence that life’s origins from non-life owes to a process of physics and chemistry. I have explained some of that evidence on this website before on multiple occasions, for example here and here.