...... or should that be Eocenes, or are they ancient aliens?
Review of all 7 episodes:
This very impressive story has a prehistoric but advanced race of reptilian beings becoming involved in a confrontation with humans having been in a state of suspended animation for millions of years. They are named Silurians by a scientist but in a future story The Doctor asserts Eocenes would be a slightly more likely description of the era of history in which they originate. However, while it is stated they were on Earth long before humans it is not stated that they originate from Earth. It would make more sense if they are aliens who evolved on another planet and arrived on Earth many millions of years ago. The Doctor does refer to them repeatedly as "aliens".
These 'Silurians' have returned in recent Doctor Who, one even becoming a close ally of the 11th and 12th Doctors, but look very different in this original appearance. They are well realised (although the dinosaur they keep like a guard dog is an unnecessary and less successful addition) and the whole story is thoroughly absorbing.
This adventure is cleverly written by Malcolm Hulke with fine scripts and an intelligent plot, the only big problem in the writing is one bad scientific blunder where the Doctor theorises that an object the Silurians saw on collision course with Earth millions of years ago was captured by the Earth's gravity and became the Moon. Modern science estimates the Moon has been in Earth's orbit for 4 to 4.5 billion years! Even the very ancient Silurian period, when moss like plants and small arthropods are the only known life to have existed on land, began 443 million years ago. That was 3.5 to 4 billion years after the Moon came into orbit. Early humans only came into existence 1 or 2 million years ago and the Silurians say this object was coming towards Earth AFTER they had been co-existing with humans so the Doctor would be making an uncharacteristically horrendous bit of historical and scientific judgement in stating it was the Moon.
That one blunder in episode 5 drops that otherwise excellent episode down in my estimation but otherwise the writing is top notch with very intelligent ideas and smart dialogue. The whole story is very well acted. Jon Pertwee, Nicholas Courtney, Caroline John, Peter Miles, Fulton Mackay and Geoffrey Palmer are all superb. The story is also very nicely filmed and has some cracking scenes throughout. There is a great moral theme underlying the story of whether to deal with a threat by peaceful negotiation or by military means. A theme just as relevant today as it ever was. The Brigadier and the Doctor are put on opposite sides of this debate which adds greatly to the moral dilemma the audience has to consider.
Apart from that one glaring line of dialogue regarding the Moon there are only really two other small minus points. Firstly the incidental music by Carey Blyton, which has silly and annoying kazoo sounds recurring. Secondly, a few of the effects which were not convincingly realised such as the dinosaur and the Silurians scorching their way through walls in episode 7. But bearing in mind the limitations of age and budget this is very forgivable. I would ideally have cut the superfluous dinosaur and the line about the Moon entirely and changed the way they entered the research centre. This is a terrific story and well within my top 100 but it could possibly have been a top 30 story, for me, with a few issues ironed out, particularly the scientific error about the Moon which drops episode 5 in my ratings.
Pertwee begins to establish himself nicely after his strong debut and Courtney and John as the Brigadier and Liz Shaw build upon their already engaging characters whilst already developing a little depth with the Brigadier showing a slightly darker side. The alien plague subplot neatly added into the mix later in the story reignites interest and drama and provides some of the best scenes such as Masters (Palmer) inadvertently spreading the plague in London and Dr. Lawrence (Miles) going berserk in episode 6.