I hadn't given much thought to this question. In fact, I believed that the decathlon was just an expanded pentathlon. The modern version of the latter was fixed at the 1912 Olympics. (Here I am referring to the pentathlon in athletics and not to what is known as "modern" pentathlon. I will devote some future post to the modern pentathlon which has nothing "modern" and is just a combined event suitable for 19th century army officers). In fact the pentathlon was revived at the 1906 Athens intercalated Olympics in its ancient version consisting in a standing long jump, greek style discus throw, javelin throw, stadium run, and greco-roman wrestling. The formula was promptly abandoned (all the more so since there were suspicions of an arrangement between the two Swedes in the wrestling match resulting in the victory of Mellander, Lemming having already secured the gold medal in javelin throw). In the 1912 Stockholm Olympics wresting was replaced by a 1500 m run and the classification was based on places. The pentathlon was short lived since it was included in the program of just two more olympiads, 1920 and 1924, only to disappear definitely after this.
The decathlon made its appearance in the 1904 Olympics (but Zarnowski believes that the decathlon was not an official event of the Olympics). The events were: 100 yard dash, 16 pound shot put, high jump, 880 yard walk, 16 pound hammer throw, pole vault, 120 yard hurdles, 56 pound weight throw, long jump and one mile run. It was contested over a single day. (There was also a triathlon in the program of 1904 consisting in long jump, shot put and 100 yard dash. That was the only occasion such an event appeared in the olympic program). The 1908 did not comprise any combined event. Then in 1912 the decathlon made its appearance in the two-day form it is contested for over a century now. In fact, in 1912, and only that once, the decathlon was contested over three days. The events were: 100 m, long jump, shot put, high jump, 400 m for the first day and 110 m hurdles, discus throw, pole vault, javelin throw and 1500 m for the second. Clearly there is a substantial overlap between the 1904 events and the 1912 ones. I found the answer as to the origin of these events in the book of the great combine events specialist Frank Zarnowski, "All-around Men, heroes of a forgotten sport"
If you are interested in the prehistory of modern athletics this is a book you should absolutely read. One learns, among others that the amateurism was a short-lived phenomenon. Track and field has been a domain where professionals have excelled in the 19th century. It was one of those professionals, Donald Dinnie, who first proposed a man-to-man contest (accompanied by a substantial prize) comprising the following events: putting the heavy stone, throwing the heavy hammer, tossing the caber, wrestling, leaping, putting the light stone, throwing the light hammer, throwing the 56 pound weight and running (an enneathlon!). The influence of Highland-Caledonian culture is obvious in the choice of events. Gradually these competitions became more frequent and by 1890 the program of events and the order were standardised. It was precisely the very same as the one of the 1904 St. Louis Games. The all-around competitions started having success in Europe as well but over metric distances and by replacing the race walk by a 400 m or a triple jump. Entering into the 20th century the swedish federation included the decathlon in its annual competition programme. It was a competition very close to the decathlon as we know it, using scoring tables. The events were: high jump, 100 m, shot put, long jump, 110 m hurdles, discus, triple jump, javelin, pole vault and 1500 m. Finally at the 1912 OLympics the decathlon was standardised to the current one.
So the decathlon events came form the all-around ones, which are of Highland and Caledonian origin. A European touch contributed into giving to the decathlon its present form. And in case you are wondering who has been the great all-around champion at the beginning of the century, well, he is none other than Jim Thorpe, the famous gold medalist of the Stockholm Olympics. He is the first and last person to hold world records in both the all-around and the decathlon. The international success of the decathlon was the death-knell for the all-around. The latter managed to survive in the US untill 1922 when it disappeared from the championships program. It made later a brief come back and finally, in 1977, it went definitely the way of the dodo.