Are we human because of unique traits and attributes not shared with either animal or machine? The definition of "human" is circular: we are human by virtue of properties that make us human (i.e., distinct from animal and machine). It is a definition by negation: that which separates us from animal and machine is our "human-ness".
We are human because we are not animal, nor machine. But such thinking has been rendered progressively less tenable by advent of evolutionary and neo-evolutionary theories which postulate a continuum in nature between animals and Man.
Our uniqueness is partly quantitative and partly qualitative. Many animals are capable of cognitively manipulating symbols and using tools. Few are as adept at it as we are. These are easily quantifiable differences - two of many.
Qualitative differences are a lot more difficult to substantiate. In absence of privileged access to animal mind, we cannot and don't know if animals feel guilt, for instance. Do animals love? Do they have a concept of sin? What about object permanence, meaning, reasoning, self-awareness, critical thinking? Individuality? Emotions? Empathy? Is artificial intelligence (AI) an oxymoron? A machine that passes Turing Test may well be described as "human". But is it really? And if it is not - why isn't it?
Literature is full of stories of monsters - Frankenstein, Golem - and androids or anthropoids. Their behaviour is more "humane" than humans around them. This, perhaps, is what really sets humans apart: their behavioural unpredictability. It is yielded by interaction between Mankind's underlying immutable genetically-determined nature - and Man's kaleidoscopically changing environments.
The Constructivists even claim that Human Nature is a mere cultural artefact. Sociobiologists, on other hand, are determinists. They believe that human nature - being inevitable and inexorable outcome of our bestial ancestry - cannot be subject of moral judgment.
An improved Turing Test would look for baffling and erratic patterns of misbehaviour to identify humans. Pico della Mirandola wrote in "Oration on Dignity of Man" that Man was born without a form and can mould and transform - actually, create - himself at will. Existence precedes essence, said Existentialists centuries later.
The one defining human characteristic may be our awareness of our mortality. The automatically triggered, "fight or flight", battle for survival is common to all living things (and to appropriately programmed machines). Not so catalytic effects of imminent death. These are uniquely human. The appreciation of fleeting translates into aesthetics, uniqueness of our ephemeral life breeds morality, and scarcity of time gives rise to ambition and creativity.
In an infinite life, everything materializes at one time or another, so concept of choice is spurious. The realization of our finiteness forces us to choose among alternatives. This act of selection is predicated upon existence of "free will". Animals and machines are thought to be devoid of choice, slaves to their genetic or human programming.
Yet, all these answers to question: "What does it mean to be human" - are lacking.
The set of attributes we designate as human is subject to profound alteration. Drugs, neuroscience, introspection, and experience all cause irreversible changes in these traits and characteristics. The accumulation of these changes can lead, in principle, to emergence of new properties, or to abolition of old ones.
Animals and machines are not supposed to possess free will or exercise it. What, then, about fusions of machines and humans (bionics)? At which point does a human turn into a machine? And why should we assume that free will ceases to exist at that - rather arbitrary - point?