Before trying to answer that question, I think it's important to ask what an answer to the question might look like and what possible consequences there could be in answering it.

Firstly, what are we looking for in an answer to this?

"Human" is just a word. A two-syllable English word that serves as a label for some grouping.

The boundaries of that label can be defined as something that belongs to the set of things that are classified by some system as belonging to the grouping (where that grouping is meaningful in terms of its relationship to other concepts/ideas).

These bring up a few follow-up questions. Firstly, what is the system in question? What properties does it exhibit? To what degree is the system self-consistent in its labeling when related to a variety of other concepts/ideas? What purpose does the labeling as specified serve?

Some labels are highly unambiguous and self-consistent. For example, the definition of a circle is defined with great mathematical rigor where the necessary and sufficient conditions for a circle are clearly laid out. There can be alternately worded definitions which yield exactly the same results, but there is essentially no real controversy on the point of what counts as a circle in mathematics. It's really just a tool for communication between people and there isn't any real obvious reason for someone to push a new definition that runs in contradiction to the standard definitions. Having it defined one way or another may be preferable for reasons of simplicity or aesthetics, but for the most part it probably won't really change how people think they should deal with circles (although it might allow them to more easily make connections that were previously harder to make by setting their mind in a certain context).

A word like "human" doesn't share the same degree of unambiguous clarity. Different people attach different meanings to the word and often the same person means different things by the word at different times in different contexts. So what is it that you're looking for? A random person's arbitrary stab at a definition at the point in time when they're asked about it? People don't really think of humans in terms of their necessary and sufficient conditions when they talk about humans, they think of a vaguely bounded cluster of things that share similar characteristics when relating to some small sampling of other concepts. What that sampling is and how they feel about those relationships make all the difference. In practice, the definitions they give you will probably be a definition that fits a particular agenda meant to shape how you treat entities which they think of as roughly falling in the same cluster of things they think about as humans in the particular contexts they have in mind. However, if you question them about edge cases and different contexts they will either start contradicting themselves or their definition will start to look either arbitrary, inconsistent with your and other's definitions/intuitions, or both.

I think ultimately the answers to this will illuminate things about different people and how they choose to define things, but almost nothing concrete and consistent and meaningful about the concept of a human. Because there isn't a concept of a human. There are many concepts which happen to share the same word because they're not that far apart in terms of the set they capture as belonging to them for most practical day-to-day purposes. Instead of trying to find the one true definition of what a human is (which doesn't really exist), I think it's more useful to think about why you care about the label of what counts as a human in a certain context. Then you can have more substantive discussions on the real matters you care about without letting the inherent fuzziness of certain labels distract us from the matters at heart. When there is a many-to-one relationship between concepts and a label/word, arguing about the label tends to get in the way. It stops being about real and important philosophical questions and becomes about pushing agendas through the use of language manipulation. Instead I think it's more useful to talk about the properties and characteristics of things/entities and how they relate to the matter of inquiry.

So perhaps a human sort of "ceases to be a human" in terms of the language we use for practical purposes when the entity no longer seems to meaningfully share the characteristics relevant to the issue/context at hand that the large majority of other entities in the cluster of things we call humans seem to share, but because any two people talking about a particular issue are probably operating with different criteria and groupings and after-the-fact definitions in mind they need to be careful if they don't want to talk past each other on the topic. (To further complicate the matter the characteristics are often on some spectrum instead of being a strict binary so you some definitions will allow for degrees of "human-ness")