Was there a single ancient religion that all others sprang from?

There are lots of religions that was inspired by others.

Roman mythology is in fact almost fully taken from the Greeks. There are some differences, like gods' names and heroes (Ulysses vs Oddyseus), and the main difference is the legend of foundation of Rome (Romulus, Remus and she-wolf).

In the Acts of Apostles chapter 17 there is a story about St. Paul in Athens where he finds an altar "to the unknown god" and he uses this to introduce Christian God to people. This altar was made because there where many cultures (Persian, Egyptian etc.) having their own gods, which were not treated as wrong by the Greeks, but some kind of supplement to their pantheon. I think many foreign gods became also Olympian gods, for example there was always a god responsible for some territory or terrain feature (like gods of rivers, forests etc.). They were accepted while visiting new countries.

And of course the idea of one god (God) is an Israeli development, which was then taken (or improved) by Christianity and then by Islam. Combination of many religions led in later times to creation new ones, like eg. Voodoo.

However, answering to your question. Some religions state, that they were revealed by god(s). In fact, the latest sources of the religions are holy books of themselves and there is no written moment "ok, for now we were atheists and we begin to believe that the truth is out there".

We can only imagine what is the reason of "creation" of gods, of course if they had not existed forever. The natural forces which were not to be explained by small brain, were inspiration of gods. There are lots of natural forces that have (good or devastating) impact on the environment (like rain, thunderbolt, grow of flora, large animals, astronomical events etc.), or on the human himself (like fire that is hot, death of old age). I think that primitive man seeing these random events must have thought that there is some kind of supernatural control in this.

I don't know when this happened, but I think this is the answer to your question: the oldest religion of man is the Nature itself. Because it is much stronger than human, not controllable, not explainable and sometimes acting like it was pleased or angry, this can be considered by the oldest god.

In every (I think) religion gods can control the Nature, like an archer controls his arrow, spearman controls his spear, a farmer controls his corn, a shepherd controls his flock. This control seems to be such obvious, that nothing in the world should have not been left without control. The feeling that everything has its controller fulfils one of human basic needs: the need of security.

Well, the obvious answer is that both Christianity and Islam emerged from Judaism. Christianity is pretty easy to see, since most of us in the West have at least a passing familiarity with the documents (particularly the New Testament) which note the early consolidation of a particular apocalyptic Jewish death cult into what we now know as the Christian religion.

The direct link between Judaism and Islam is a bit more tenuous and, given the poor relationship between the adherents of those religions right now, controversial. However, Wikipedia notes that there was at the very least a bit of tolerance between Mohammed and the earlier religions:

http://en.wikipedia.org/wiki/Islamic%E2%80%93Jewish_relations#Religious_figures

In the course of Muhammad's proselytizing in Makkah/Mecca, he initially viewed Christians and Jews (both of whom he referred to as "People of the Book") as natural allies, sharing the core principles of his teachings, and anticipated their acceptance and support.

I read a narrative a few years ago whose name completely escapes me that hid a fairly plausible explanation for this camaraderie: that Islam itself arose from a coalition of Arabs and messianic Jews, and that while Muhammed himself may or may not have been born Jewish, he based much of Islam on Jewish teaching. It was an interesting book, wrapped up in the narrative format because it was aimed at the general public rather than academia.

Otherwise, I've heard people (not all of them evangelists) insist that LDS is not a Christian religion, and if you believe this to be the case, then that is most certainly an example of a large religion which has its basis in another one. There is no question that Joseph Smith drew heavily on the Old and New Testaments for inspiration.

If you read the Old Testament, you will see hints of polytheism being at the roots of Judaism. Comparing the different names you see this as well.

The story of Abraham choosing one god above the others is a case in point. El, a Canaanite god name etymologically related to Allah, is generally the original word for God in Judaism. After Moses, however, you have that and YHWH. It was written this way to hide the true name. One theory I read is that Moses actually picked up this name from his father-in-law when he was in exile and combined it with the Hebrew El. It is my opinion that Elyah sounds a lot like Allah.

As the nations develop, there are lots of references to other gods getting in the way and causing El's anger against a king. Even later in the Old Testament when they were in exile in Persia (practicing monotheistic Zoroastrianism) you see the leaders of Persia and the Hebrews talking about God as if it's the same one as their own. Persia's religion is derived from the Indo-European religions where Zoroaster also chose one god above the others, Ahura Mazda over the Devas.

Note in Hinduism it's the reverse, the Devas beat the Asura. The Hindu Asura, being composed of gods like Dyaus Pita (sky father) which is etymologically equivalent to Zeus Pitar (Greek), Jupiter (Roman), and Tiu (Norse/German) which Tuesday is named for. Asura is etymologically related to Aesir and Asgard from Norse.

Buddhism is an offshoot of Hinduism. It is fairly clear Hinduism/Buddhism is derived from the the same roots ultimately as Greek, Roman, Norse, and Zoroastrianism.

Christianity and Islam are derived from Judaism. Less clear is the influence of Zoroasterianism on the Judaism/Christian/Islam monotheism. The Persian connection is interesting. One possibility is monotheism didn't actually exist in Isra-El and Judah until the Persians influenced them.

In that case it's possible that all of these major religions are partially derived from the base Indo-European pantheon with heavy influence from Semitic polytheism (Canaan, Egypt, and Babylon). Another possibility is Abraham, Moses, and Zoroaster independently came up with monotheism. Even later people such as King Josiah may have. It's probably a big jumble of influences with one group of people deciding there is only one God.

My sources are reading the English Bible, Wikipedia, and Online Etymology.

No I dont think we could trace it all back to one religion. Religion is a big part of the human experience the need to create a mythical realm where we go after death, seems to be a part of just about every culture. The major religions of today are likely derived from several older religions.

Religions are cultural concepts, they evolve through the time, adapting some ideas from others, providing some new ones, etc. For example, having Christianity of 500 AD, you'd be able to point out some concepts taken (directly or not quite so) from Judaism, Zoroastrianism, Neoplatonism, etc. Then, Christianity itself influenced Islam, Manicheism, lots of Gnostic denominations. Putting these influences graphically, the graph would be in my opinion pretty dense.
The answer to the question would be, that yes, we'd be able to point out some religions, like Judaism, Zoroastrianism, hypothetical Proto-Indo-European cult, etc., which had greater influence than others, and developed concepts used later by many others through the time. But, whose influence was greater, more inspirational, etc., that's highly speculative and controversional, hence - there will be no concrete answer.