|Left: an existentialism puzzle pieces (Ѻ) cartoon, where each piece is part of the big picture of the solar system (or universe). Right: an existentialism image (Ѻ) from Joe Pellegrino’s webpage on an “Introduction to Existentialism”.|
The works of Blaise Pascal and later Soren Kierkegaard, according to Jennifer Hecht (2003), considered as forerunners of existentialism. 
In c.1642, Pascal, in his Thoughts notebook, circa aged 19, penned the following views on the uncertainties of existence:
“I know not who put me into the world, nor what the world is, nor what I myself am. I am in terrible ignorance of everything. I know not what my body is, nor my senses, nor my soul, not ever that part of me which thinks what I say, which reflects on all and on itself, and knows itself no more than the rest. I see those frightful spaces of the universe which surround me, and I find myself tied to one corner of this vast expanse, without knowing why I am put in this place rather than another, nor why this short time which is given me to live is assigned to me at this point rather than at another of the whole eternity which was before me or which shall come after me. I see nothing but infinities on all sides, which surround me as an atom, and as a shadow which endures only for an instant and returns no more. All I know is that I must die, but what I know least is this very death which I cannot escape.”
In c.1830, Kierkegaard stated the following "why of existence" very-humorous stylized query:
“Where am I? Who am I? How did I come to be here? What is this thing called the world? How did I come into the world? Why was I not consulted? And if I am compelled to take part in it: Where is the director? I want to see him.”
Later oft-classified early period existentialists include: Leo Tolstoy, Fyodor Dostoyevsky, Friedrich Nietzsche, and Jean-Paul Sartre.
In c.1950, Albert Einstein engaged into a Q&A style dialogue (see: Einstein-Pascal dialogue), with an anon 19-year-old American engineering student, on the topic of:
“What is the purpose of man on earth?”
platformed on Pascal’s earlier Thoughts notes.
|A screenshot from a 2015 video (Ѻ) on Jean-Paul Sartre and existentialism.|
In 1977, American physicist and open atheist Steven Weinberg, in his The First Three Minutes, outlined the generally-accepted scientific model of the early minutes of the start of the universe, according to big bang theory, at the end of which he concluded:
“The more the universe seems comprehensible, the more it also seems pointless.”
Here, reading between the lines, or elaborating on this “chilling effect” stylized message, it would seem, according to the Weinberg model (compare: Camus model; Gates model), that existence, in an atheistic universe, is “pointless”. This assertion, naturally enough, generated much debate and reaction in the decades to follow (see: pointlessness).
In 2002, Philip Stokes, in his Philosophy 100: Essential Thinkers (see: Stokes 100), listed the following seven top existentialism philosophers: 
1. Soren Kierkegaard
2. Friedrich Nietzsche
3. Edmund Husserl
4. Martin Heidegger
5. Jean-Paul Sartre
6. Albert Camus
7. Simone de Beauvoir
The following are top 20 greatest existentialism philosophers, per 2017 Google search return, per order of rowed photos returned:
1. Jean-Paul Sartre
2. Soren Kierkegaard
3. Albert Camus
4. Martin Heidegger
5. Friedrich Nietzsche
6. Simone de Beauvoir
7. Georg Hegel
8. Maurice Merleau-Ponty
9. Karl Jaspers
10. Gabriel Marcel
11. Martin Buber
12. Paul Tillich
13. Emmanuel Levinas
14. Walter Kaufmann
15. Lev Shestov
16. John Macquarrie
17. William James
18. Colin Wilson
19. Henry Thoreau
20. Robert Solomon.
Karl Jaspers, to note, argued for belief in god, akin to what Kierkegaard had discussed, namely thought "absurdist irrationalism", which is what Jennifer Hecht calls "a leap". 
|A plot of a typical human reproduction reaction, wherein an initial newly formed (collided) couple, in an initial state, progress, over time (or reaction extent), over the activation energy barrier, into the final state of a tri-human-ide molecule (see: trihumanide molecule) formation, barring discussion of irregular product formations (e.g. single mothers, single fathers, parentless children, etc.)|
In modern terms, in a big bang based, god-less, spirit-less, soul-less, karma-less universe, a human comes into existence, as does any other "thing" in the universe, in the form of a "bound state" temporally-existive entity, the description of which being dependent on the state of the environment of the thing, e.g. higher temperatures tend to yield sub-atomic particle physics descriptions, whereas cooler temperatures tend to yield atomic-range descriptions.
The existence of humans, being atomic-range description things, yield to a description wherein a new human (baby), as a newly synthesized product of the universe, comes into existence at the location of a free energy minimum (see: free energy), on a plot of Gibbs energy vs extent of reaction, in the purview of a human reproduction reaction equation.
In this "sense" (see: Pascal), one's individual existence in the universe, contrary to Weinberg model, is not "pointless", but described by states of existences (existence states) quantified Gibbs energy inflection points (compare: Gates model vs Camus model).
The following are related quotes:
“Existentialism is a chiefly 20th century philosophical movement embracing diverse doctrines but centering on analysis of individual existence (see: individualism) in an unfathomable universe and the plight of the individual who must assume ultimate responsibility for his acts of free will without any certain knowledge of what is right or wrong or good or bad.”— Anon (2000), Meriam-Webster Collegiate Dictionary
“Existentialism is a philosophical theory or approach that emphasizes the existence of the individual person as a free and responsible agent determining their own development through acts of the will.”— Anon (2017), Google Search Definition
1. Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (pg. 457). HarperOne.
2. (a) Pascal, Blaise. (1662). Thoughts (Pensees) (pg. #). Publisher.
(b) Pensees – Wikipedia.
3. Weinberg, Steven. (1977). The First Three Minutes: a Modern View of the Origin of the Universe (pointless, pg. 154). Basic Books.
4. Stokes, Philip. (2002). Philosophy 100: Essential Thinkers. Enchanted Lion Books.
● Existentialism – Wikipedia.