One obvious thing you'd see straight away as you walked down the human line would be the height and age of the mothers - they would get progressively shorter and younger wouldn't they?
Well, this depends who you are and what time period you're speaking. Given that you contrasted it to millenia for the chimps, I guess you mean a time span of centuries. Not all populations of humans have gotten bigger over the last century or so, and the change isn't going to continue indefinitely into the past. You'd probably find a fluctuating height, due to individual variation, changes in nutrition as lifestyle changes, and changing ethnicity.
The idea that humans were all shorter, then 20th century nutrition suddenly shot them up in height is dubious. An analysis of the 43 people buried at a mass grave near the Battle of Towton (1461, for those whose British history isn't so hot) showed that their height varied from 1.5 to 1.8 metres tall. This puts them in the range of average height for English men today, and leaves the average a bit taller than men in 18th century England. These things vary.
On the chimp line, nothing much would change except over millennium.
I think a lot more would change than you think. We have a tendency to underestimate the level of diversity in other apes, but not all chimps are the same size, and their skin varies in colour. You'd probably find racial shifts in the chimp population as much as in the human one.
Anyone know off-hand where the parallel lines meet - pan prior? - what it would look like and where it would live? (Meanwhile, Google is my friend..)
Now that's a difficult question to answer, except for the 'where would it live' bit, and there I have no idea except 'Africa'.
Hmm - that's interesting and a little disappointing :-)
Could it be that those involved in the battle were warriors and therefore larger? I really don't want to believe that all those medieval buildings had doorways too small for the people living in them!
Is there any evidence that modern man gets taller with improved nutrition? Are Asians in fast developing countries like India and China getting taller?
Sorry for the slow reply! Improved nutrition in early life does lead to taller populations usually. The misconception is the belief that nutrition has steadily improved over human history, so you get shorter the further back you go. These warriors in mostly agricultural 15th century England properly ate healthier than the slum dwellers who made up a sizable portion of the population of industrial 18th century England. The ups and downs of your ancestor's height would depend on which societies and ethnicities you're tracing that ancestry through.
It would simply demonstrate, with evidence, how both lines developed from a common ancestor and it would have been quite a powerful story that is fairly easily explained.
I wasn't expecting it to change all creationist's minds - particularly the ones found on these boards.
I seem to remember that the horse has a pretty good fossil record - anyone know what its nearest living relative and common ancestor are? ["Hello Google"]
The nearest living relatives of horses are zebras and asses. They're very close relatives, all classed in the same genus, and horses are believed to have split from the rest only about 2.5 million years ago. There is an extensive fossil record of the horse/ass/zebra genus Equus, with dozens of extinct species being named.
However, despite this, there is nothing we can point to as the common ancestors of horses and asses/zebras, and there never will be. The simple question is, how could we possibly ever know? Think back to crashfrog's analogy of family relationships. You and your cousin share a common ancestor, your grandfather. Imagine you then come across pictures of your grandfather, your grandfather's brother, and your grandfather's cousin. All of these may share some distinct family traits, and in all of them you might be able to see the resemblance. Without some sort of documentation, however, you would never be able to tell which one was your grandfather, or even if none of them are and your grandfather's picture was lost.
And that's similar to the situation we're looking at in fossils. We can identify the family relationships. You can dig up a load of fossils of horses and their relatives, and you can tell that they're all relatives. If the material is good enough, it's possible to make pretty good family trees. But there will never be a way of distinguishing between a (metaphorical) grandfather, grandfather's brother and grandfather's cousin.
Having said all that, if you want the scientists' best guess as to the fossil nearest to the horse/ass/zebra ancestor; the answer is Equus simplicidens from Idaho about 3 million years ago.
If your link means the picture posted above when it talks about the 'Evolution Ape Man chart', then it seems strange to talk about trying to fill it in in 1925, since these drawings are taken from an illustration produced in 1965 by Rudolph Zallinger.
What's more, it seems odd to talk about the search for fossils to fill in this chart, since the pictures are all based on actual fossils. In the abridged version you posted above, the first two appear to be, in order Dryopithecus, first found in 1865 in France; and Ramapithecus, first found in 1932 in India (and since reclassified as Sivapithecus). I'm having difficulty identifying the rest, since the pictures don't appear to be an exact copy of the original drawing, but you can look at the full list of illustrations from the original picture at Wikipedia if you like. No Nebraska Man, no Piltdown Man, no Java Man.
As for the tired old claim that all known fossils are either only ape or only human, rise up to the challenge posted earlier and demonstrate this. Explain what the clear difference between ape and human bones are and apply it to the fossils posted above. I can guarantee that whatever arbitrary definition you come up with will leave you with some apes and humans more similar to each other than they are to their supposed brother apes/humans.
I think your ring species example has a bit of a problem. Firstly, because Siberia is east of Britain, not west! But, more importantly, the herring gull doesn't appear to be a ring species, according to DNA evidence. The history of herring gull speciation is very complex, according to the evidence from mitochondrial DNA, but Larus occidentalis of the American west coast seems to be the sister group to the rest of them, so these split off from all other herring gulls first. The rest fit into two family groups, one of which forms an incomplete ring that stretches from Western Europe, through Russia and into the Americas, but which doesn't cross the Atlantic to form a complete ring; and the other of which is found in Europe and North America. You can read the DNA study online for free here.
For a better example, try an old favourite of this board - Asian Greenish Warblers.