Unbridgable gap model
A rendition of the unbridgeable gap model: the thinker who holds-fast to the ancient mythological doctrines of 'life', 'soul', 'consciousness', 'free will', 'choice', a 'brain', etc., will argue, to their grave, that, in some contrived-way or another, at one particular second in time, via "emergence" (the new term for intelligent design), in the course of human evolution mechanism, that molecules, somehow, came to life, acquired souls, developed a free will, obtained the a state of consciousness, evolved the ability to think, among other now-defunct traits that do not apply to the hydrogen atom, nor to any other molecule, known in science.
In science, unbridgeable gap refers to the perceived separation or boundary, e.g. materialism/physicalism vs. spiritualism/psychology, or life vs. non-life, etc., in the minds of many, that exists between the chemical world and the human world.

Overview
In 1873, French locomotion physiologist Etienne Marey published his Animal Mechanism, of which the following is the bold opening paragraph:

“Living beings have been frequently and in every age compared to machines, but it is only in the present day that the bearing and the justice of this comparison are fully comprehensible. No doubt, the physiologists of old discerned levers, pulleys, cordage, pumps, and valves in the animal organism, as in the machine. The working of all this machinery is called "animal mechanics" in a great number of standard treatises. But these passive organs have need of a motor; it is "life", it was said, which set all these mechanisms going, and it was believed that thus there was authoritatively established an inviolable barrier between inanimate and animate machines.”

Marey goes on to say that because modern engineers have created machines, which via combustion, and the animation of various mechanical organs, thereby, we are now more legitimately able to compare animals to animate motors, we now need to “seek another basis for such distinctions”.

In 1875, German physician and diplomat Ernst Gryzanowski stated the following in his dissection of the social physics of Auguste Comte: [9]

“We maintain, that as long as we are unable not only to bridge over the gulf that separates organic from inorganic nature, but even to see the bottom of it, the onus probandi must lie on those who deny its width and its depth. Comte himself holds curious opinions on this point. He deprecates the inquiry into the origin of organic life as a useless speculation (thus admitting the partly speculative nature of biology). He believes in the immutability of species, and separates, in his pedigree of sciences, biology from physics by a twofold division, making them agnates rather than cognates. It would therefore have been easy for him to admit the specific difference between man and atom, and to find this difference in self-consciousness, and in the moral freedom of man's will.”

In 1895, French sociologist Emile Durkheim stated the following: [10]

“If we begin with the individual, we shall be able to understand nothing of what takes place in the group. In a word, there is between psychology and sociology the same break in continuity as between biology and the physico-chemical sciences. Consequently, every time that a social phenomenon is directly explained by a psychological phenomenon, we may be sure that the explanation is false.”

This rather contentious statement has since been requoted by and discussed by those including: Werner Stark (1962), Edward Wilson (1977), and Michael Ruse (1984). [11]

In 1925, American physical chemist Gilbert Lewis, in his "Anatomy of Science" lecture, end chapter: “Life; Body and Mind”, stated the following, in what seems to well-capture the gist of the 2010 inception of hmolscience: [8]

“Perhaps our genius for unity will some time produce a science so broad as to include the behavior of a group of electrons and the behavior of a university faculty, but such a possibility seems now so remote that I for one would hesitate to guess whether this wonderful science would be more like mechanics or like a psychology.”

Lewis, further on toward the end of his lecture, stated to the following:

“We cannot forget that there are two kinds of behavior with which we are already intimately acquainted: on one hand, the behavior of weights and electric charges and chemical reagents; on the other hand, the behavior of man. These require two distinct vocabularies, and most writers who describe animal behavior have adopted the one or the other. We have ‘nature fakers’, who make animals think and act just like men, and there are the others, who regard the swarming of bees as a sort of chemical reaction. I do not know which of these two extremes to regard as the more futile, for both extrapolations go far beyond what is now justifiable. Yet the attempt to bridge the vast gulf is a legitimate aim of science.”

In 1936, Russian plant physiologist Alexander Oparin, in his The Origin of Life, stated the following: [12]

“Neither the theory of spontaneous generation nor the theory of the continuity of life solves rationally the problem of the origin of life, since these theories are based on the tacit assumption of an absolutely impassable hiatus between animate and inanimate nature.”

In 1938, English physiologist Charles Sherrington in his “Man on his Nature” lecture turned book was the first to state clearly that there is no boundary, that the various divides (e.g. life/non-life) or dualisms (e.g. mind/matter) are fictional: [7]

“In the middle ages, and after them with Fernel, as with Aristotle before, there was the difficulty of the animate and the inanimate and finding of the boundary between them. Today’s scheme makes plain why that difficulty was, and dissolves it. There is no boundary.”

In 1953, American economist Robert Heilbroner stated the unbridgeable gap viewpoint as follows: [1]

“There is an unbridgeable gap between the ‘behavior’ of [subatomic particles] and those of human beings who constitute the objects of study of social science. Aside from pure physical reflexes, human behavior cannot be understood without the concept of volition—the unbridgeable capacity to change our minds up to the very last minute. By way of contrast, the elements of nature ‘behave’ as they do for reasons of which we know only one thing: the particles of physics do not ‘choose’ to behave as they do.”

In 2001, South African chemical physicist and human free energy theorist Adriaan de Lange stated the following in regards to recollection of a once-held gap mindset:

“Since as a kid I was aware of the abyss (super rift) between the material and mental worlds. I tried to bridge this abyss, but could not. Yet I kept on with my own "Steigerung" as Goethe did. Then during 1982-83 I discovered empirically that the law of entropy production applies to the humanistic world as it applies to the physical world. My joy knew no bounds. I have found the bridge between these two worlds with which to cross the abyss between them. Others thought I was crazy and would not dare to publish my account.”

In 2002, Turkish-born American physicist Turner Edis gave the following statement on the matter: [4]

“Even in the realm of what we see, there must be unbridgeable gaps separating order from chaos, animate from inanimate, life from nonlife, humans from other animals, and consciousness from nonconscious matter.”

It is difficult to see, to note, whether or not Edis is for or against this statement because he footnotes this to two religious themed books (Arthur Custance’s 1978 Science and Faith, pg. 134-49, and Gay Habermas and J.P. Moreland’s 1992 Immortality: the Other Side of Death, pg. 51) and mentions how some religious philosophers still defend the notion of an animal soul or life force irreducible to physics. [4]

Life vs. Non-life
Variations of this perceived 'gap' separating humans from chemicals are captured in the various "what is life? debates" along with debates on differences between life vs non-life, animate matter vs inanimate matter, living vs non-living, organic life vs inorganic life, vegetable life vs animal life, human life vs animal life, etc.
Life non-life gradient
Danish chemist Martin Hanczyc, in his 2001 TED talk “The Line Between Life and Not-Life”, argues that there is no longer an unbridgeable gap (or dividing line), but rather over the last 150 years or so “science has blurred the distinction between non-living and living systems”, as he diagrams above, which is nothing but superficial recourse to a mix of emergent hypothesis and properties hypothesis: namely that certain properties constitute "life" and that these various properties have "emerged" over time (and or a life principle argument). [5]

Vernadsky’s gap
An interesting twist on the gap between the life and non life perspective or dichotomy is Russian biogeochemist Vladimir Vernadsky’s 1926 perpetual life model of living matter, in which he argued that living matter or biosphere was comprised of certain specific elements and that the remainder of the azoic parts of the earth, such as the crust or atmosphere were composed on inert matter, and that this imagined separation of the two types of matter has remained intact since time eternal and that living matter cannot be created from non-living matter or that abiogenesis is impossible. In his own words: [2]

“It is very clear that living matter becomes manifest without abiogenesis. In other words, living organisms have always sprung from living organisms during the whole geological history; they are all genetically connected; and nowhere can solar radiation be converted into chemical energy independent of a prior, living organism. We do not know how the extraordinary mechanism of the earth’s crust could have been formed. This mechanism is, and always has been, saturated with life. Although we do not understand the origin of the matter of the biosphere, it is clear that it has been functioning in the same way for billions of years. It is a mystery, just as life itself is a mystery, and constitutes a gap in the framework of our knowledge.”

What is interesting here is that Vernadsky readily admits that he has discarded religious views and moreover that he is an adherent of thermodynamic models, physics, gas system models of the biosphere, but the origin of life, for him remained somewhat of a mystery. To note, Vernadsky’s ‘theories on perpetual life’ came under attack by Russian biochemist Alexander Oparin who stated that they were not in accord with the “objective data of modern science.” [3] Some have noted that latter in life, Vernadsky began to recant his gap view and concede that life could have had an origin. [2]

Mechanism view | No gap
See also: Defunct theory of life
The central issue is embedded in evolution theory, which states that humans evolved over time from earlier forms of life, as popularized in large part by English naturalist Charles Darwin and his 1871 warm bond model, which argues that life began in a heated pond 3.9-billion years ago. Taken at face value, this is a cogent argument, yet the inquisitive thinker is soon led down the path to the question of if this theory is true, then at what exact millisecond in time past did the first "form" of life (whatever it may be) solidify or come into existence? If one holds to both the theory of evolution and the theory of life, the resultant paradoxical picture, shown above, will emerge.

The typical (non-religious) person will still-hold fast to the view that yes "humans evolved from earlier forms of life", with the implicit assumption that they are alive, that bacteria were alive 3.9-billion years ago (and block out the rest of the answer), and pull-out of the question at this point, satisfied with some type of emergence theory explanation (or whatever theory happens to be in vogue in that decade).

Correctly, any given molecule, such as a hydrogen molecule a water molecule a bacteria molecule or human molecule, cannot technically be said to be alive, any more than sun can be said to be alive. Correctly, the modern view finds that it is technically impossible to find a specific day (or rather second) in the contiguous chemical synthesis mechanism, on the evolution timeline, starting with hydrogen reactants H (13.7 billion years ago), stepping through a number of molecular species intermediates MI, and ending with modern human molecule products MH (200,000 years ago):

In modern view, every intermediate, MI1, MI2, MI3, etc., in the steps of chemical synthesis of the human molecule, over time, are simply only bigger-and-bigger, more-and-more dynamic molecules, derivative of the hydrogen atom. Subsequently, one is forced into one of two conclusions, either the hydrogen atom is alive or the human molecule is not alive. The former is nonsensical. Others, has history has shown, will argue that these olden-days properties said to be characteristic of "living beings" (living matter, living system, living organism, living molecules, ), are "emergent", or "self-organizing", or "auto-catalytic", etc., properties, or in possession of some type of "living energy" (or living force), or in a specific "living state", etc., and write entire books and spend decades in attempts to salvage the olden-days concepts.

A few prime examples, used to explain the "missing link" mechanism step (dividing life from non-life) in the above contiguous mechanism, include the 1926 theory of Vladimir Vernadsky who argues that “living matter” of the “biosphere” (sphere of life) is a type of “green fire” of stable compounds in a thermodynamics field living off of free energy; the 1970s views of Ilya Prigogine, who posits that living things are a far-from-equilibrium sort of Benard cell state or type of dissipative structure; to the 1990s views of Stuart Kaufman to argues that life is an auto catalytic reaction, able to complete one thermodynamic cycle, that somehow that ‘catches fire’.

In any event, in summary, many newcomers to the subject of the "human molecule" will object to the definition of a human being as 'molecule' on the grounds that a human being has a soul (religious objection), that humans have internal crystalline structures, e.g. teeth enamel, hence the term molecule cannot be used (technical jargon objection), that living things have an atomic turnover rate of about 48-percent of structural atoms per year and thus cannot be a molecule (theoretical issue), or that a human cannot be a molecule because a person has a brain (conceptual issue), that a human cannot be a molecule because a human has consciousness, choice, and free will (educational issues), and so on. Whatever the objection, there is no doubt that humans are made purely of atoms, meaning that humans can be categorized as a type of chemical entity using whatever name one prefers, and that humans are reactive to each other, hence the subject of 'human chemical reactions' is a topic germane to human chemistry, and subsequently, study in this fields invariably leads one into the modern view that humans are simply reactive animate molecules moving about on a surface, which acts as catalyst, driven by cyclical heat input from the sun, just as is any heat engine driven or rather worked by the alternating contact of a hot body and cold body.

References
1. (a) Heilbroner, Robert. (1953). The Worldly Philosophers, 7th ed. (pgs. 316-17). Penguin, 1999.
(b) Ball, Philip. (2004). Critical Mass - How One Thing Leads to Another (quote, pg. 207). Farrar, Straus and Giroux.
(c) The Worldly Philosophers – Wikipedia.
2. Vernadsky, Vladimir I. (1926). The Biosphere (pg. 89). Copernicus.
3. Oparin, Alexander I. (1957). The Origin of Life on Earth (pg. 59). New York: Academic Press.
4. Edis, Taner. (2002). The Ghost in the Universe: Science in the Light of Modern Science (pgs. 54, 78). Prometheus.
5. (a) Hanczyc, Martin. (2011). “The Line Between Life and Not-Life”, TED talks, May.
(b) Why can’t science make life? (2011) – TheScienceForum.com.
6. Carroll, Bud. (2002). The Materialistic Wall (thermodynamics, 5+ pgs.). Trafford Publishing.
7. Sherrington, Charles. (1940). Man on His Nature (pg. 242). CUP Archive.
8. Lewis, Gilbert N. (1925). The Anatomy of Science (behavior, pgs. 195, 199-200). Silliman Lectures; Yale University Press, 1926.
9. Gryzanowski, Ernst. (1875). “Comtism” (human molecules, social molecule, pg. 276; inorganic nature, pg. 277). The North American Review, 120: 237-80, April.
10. (a) Durkheim, Emile. (1895). Les Regles de la Methode Sociologique (pg. #). Publisher.
(b) Durkheim, Emile. (1938). The Rules of Sociological Method (translators: Sarah Solovay and John Mueller; editor: Georg Catlin) (pg. 104). University of Chicago Press.
11. (c) Stark, Werner. (1962). The Fundamental Forms of Social Thought (pgs. 240-41). Routledge & Kegan Paul.
(d) Wilson, Edward O. (1977). “Biology and Social Science”, Daedalus, 106, 125.
(e) Ruse, Michael. (1984). Sociobiology: Sense or Nonsense (pg. 192). Springer.
12. Oparin, Alexander. (1936). The Origin of Life (introduction and translation: Serguis Morgulis) (pdf) (pg. 45). Dover, 2003.

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