Our common ancestor is a group of small lizards [0] that are mostly known for being found inside moss "trees" where they seemingly starved to death, and many of their other descendents aren't especially intelligent either.
Proving a negative is a fools errand. The fossil record developed thus far doesn't support it. Additionally there is such intelligence as the Octopus which has an evolutionary split much earlier than mammals and birds.
The last common ancestor of [humans] and octopuses is a flatworm that
trawled the sea floor 750 million years ago. This is the most recent
creature that we both have a direct line of descent from – it represents
the point at which we diverged down separate evolutionary pathways. To
illustrate just how early this was, this was 80 million years before any
animal showed bilateral symmetry – the familiar body plan with a defined
top and bottom, and right and left; 350 million years before tetrapods –
the first four legged creatures that gave rise to all birds, reptiles,
mammals and amphibians – came into existence; and 500 million years before
the emergence of dinosaurs.
One would not directly. Given measures of intelligence of evaluable existing life, work backwards to find last common ancestors given evolutionary theory from the available fossil record.
This leaves a lot to be desired, in my mind. Examples: * Soft tissue organisms may not be well enough preserved to be studied. * Can't evaluate intelligence in evolutionary dead-ends that no longer exist. * Limited evolution theory may miss mechanisms of how intelligence comes about, like maybe from something akin to a shared toxoplasmosis infection among different species rather than each getting there through a random walk.
Behavior can be inferred from the fossil record. There's quite a lot of literature on this. For example inferring nesting behavior of dinosaurs from fossilized nests and where you find them. Or tool use in early hominids.
You need to define what constitutes intelligent behavior and we certainly have some of this from studies of human evolution - e.g. tool use, emergence of art, burial practices, that kind of thing.
Cephalopods are likely to have developed a high intelligence only not earlier than the Mesozoic era, significantly later than the vertebrates.
The original technological breakthrough that has differentiated cephalopods from other animals was a shell that could be filled with gas, acquiring thus a controllable buoyancy.
The early cephalopods had a lifestyle similar with the modern Nautilus, floating freely in the water and gathering the prey around, unlike the snails and bivalves that had to sit on the bottom of the sea because of the weight of their shells.
The lifestyle of most ancient cephalopods, like ammonites, did not require a great intelligence, so it is unlikely that they had developed it. This kind of cephalopods have been dominant for a few hundred million years.
That changed only after the apparition of the ancestors of octopuses and cuttlefish, which have exchanged their protective shell for a greater mobility and which have begun to live on the bottom of the sea or close to it, where the environment was much more variable and challenging for a fast moving animal than in the free water, far from obstructions. This is when the high intelligence of cephalopods has developed, sometime during the middle or even towards the end of the Mesozoic era.
On the other hand, the intelligence of the vertebrates has developed a lot after they have conquered the terrestrial environment, which was much more complex than the marine environment, sometime during the Upper Paleozoic era, probably at least one hundred million years before the cephalopods.
Also, while your quotation is grosso modo right, it has a lot of details that are very wrong.
750 million years ago there were no animals whatsoever. Such ridiculous numbers are sometimes proposed by people who do not understand that the so-called "mollecular clocks", which are based on the frequency of inherited mutations in DNA, are not constant clocks. While the frequency of raw mutations in DNA varies only very slowly in time (e.g. due to the general slow decrease in the ambient radioactivity), only a small fraction of the mutations are inherited, because most mutations have bad effects, especially in more ancient animals, which had less redundant DNA. How bad are the effects, depends on the existing competition. When there is no competition, because either a new environment has been conquered or because a catastrophe has wiped out the competition, than bad mutations may not matter and their carriers survive, so their descendants inherit those mutations (after collecting additional mutations that undo the bad effects). That is why all the divergences between animals that have occurred after catastrophes or after arriving in new environments appear like they could be extrapolated towards much earlier intersection points, which are always in conflict with fossil data.
Moreover, the common ancestor of vertebrates and mollusks was a worm, but it certainly was not a flatworm. There are several unrelated kinds of worms that are flat, but their flatness is caused by a more recent evolution. Several groups of worms have been very small at some time in their past, when they became simplified by losing partially or totally some of their organs, like the circulatory system or respiratory system. Sometime later, they have evolved again towards greater sizes, but in all cases of evolution reversals identical developments are extremely unlikely. Normally different solutions for the same problem are found. So most "flatworms" are flat because with this form they no longer need the better respiratory/excretory/circulatory systems that their ancestors may have lost.
The common ancestor of vertebrates and cephalopods was some kind of worm, which lived significantly less than 600 million years ago, during the Ediacaran, which had bilateral symmetry and which probably ate only microscopic food filtered from the sea water.
Bilateral symmetry is the symmetry that is normal for any mobile animal living on the bottom of the sea, while radial symmetry is adequate for a sedentary animal. While for echinoderms there is no doubt that their radial symmetry has evolved from a bilateral symmetry, even for cnidarians there are good chances that their radial symmetry has also evolved from a bilateral symmetry of their ancestors, after the polyps have lost mobility as adults.
Even the fixed sponges, which may have no symmetry, might have evolved from mobile ciliated ancestors.
The traditional view of evolution was that all simpler forms must be primitive and all complex forms must be derived, but now it is clear that evolution towards the maximum possible simplification for a given lifestyle is more frequent than evolution towards more complex forms. Because of that, many groups of animals that were thought to be very primitive, like some of the flatworms, may be highly evolved, but towards simpler organizations.
