I find that the pivotal disagreement between `evolutionist' and `creationist' is the issue of how species come about. The evolutionist draws a `forking tree' picture of the process, which offends the creationist's conviction that a species is an indivisible thing, created by God and separate from any other species. Properly, the fossil record gives us only a body of `dots' on a picture, each near to the line of some branch of the tree: these are the samples we have from our data (the fossil record, recording some haphazard portion of what did live in each past era). When the evolutionist's discourse is examined with care, the proposed process of speciation involves a species becoming unusually diverse, (at least) two groups emerge within the species, the groups drift apart but aren't individually unusually diverse, eventually becoming separate species. The creationist, on the other hand, has each species coming into being, spreading, showing a moderate diversity and possibly, eventually, becoming extinct. The species may be very similar to some earlier species, but I suppose the creationist views this as the creator reusing parts of an old design in designing the new.
The thing about the fossil record is that, in this model's terms, the creator seems to base the design of any given species on one prior species in the new one's geographical vicinity: where several priors seem very like the new, they in turn share a common prior. All of which seems very sensible: but from time to time leads to oddities. The standard example is the chordate eye, such as our own: this has nerve cells coming off the front of the retina, joining up in the fluid cavity of the eye (obliging them to be transparent) and plunging out through a hole in the retina (leaving us with a blind spot in each eye). It's done that way in all species based on chordate species: but could equally have the nerves come off the back of the retina, leaving the full retina with less obstacles in its visual path - as in the squids. For some reason, the creator never borrows this design breakthrough from the squids and deploys it in any chordate species. Why ?
The creationist and the evolutionist might plausibly be able to agree on a `family forest' of life, differing only on whether the branching points represent a species `splitting' or the creator starting off a new species from close to the `thread' in our diagram that represents some old species. Whether the forest has many start-points or just one is hard to tell conclusively from the fossil record, given that an apparent `trunk' (ie the base of a single tree in the forest) will arise where a species (or enough species that some have been noticed) is based on an earlier one whose bodies (for one reason or another) do not survive as fossils. For example, one might plausibly imagine the creator basing snails on some variety of slug - the snail then leaves a shell preserved in the fossil record (and, for that matter, providing the archeologist with the sign-post which says where to look for the otherwise undetectable trace of a snail body in the rock) where the species on which it was based did not. This then leaves us with a `shadow' forest, consisting of all the life that hasn't left traces in the rock: we know so little about it that our best information is inferred from our models of what we do see, and other folk might plausibly view that as no better than speculation.
Aside from gaps in the record arising from `invisible' species, we can also expect gaps from species which lasted only briefly: in an epoch where the creator was uncommonly prolific, we can expect to see few the species that lasted only long enough to render some niche habitable before some more vigorous species drove it to extinction - the longer ago the epoch was, roughly speaking, the thinner our random sample of `what lived back then' gets. The shorter a species' span (from creation to extinction), the more likely it is that we haven't seen it. We will see some fossils of transient species: but we should expect that there were far more transients that we have missed, at least whenever the record does contain apparently ephemeral species.
The creationist's view doesn't rule out the possibility of `de novo' species, not even loosely based on any prior, being created at any time: and may, indeed, look on the `pre-Cambrian explosion' as an epoch where this happened. The evolutionist, on the other hand, sees the explosion as a period of such high turn-over in species that the fossil record lacks sufficient detail to trace the family trees through the epoch: and thus finds in it no contradiction of the hypothesis that each species is based on some prior.
Strictly, we have no proof that the creator never creates any de novo species: but, equally, we have no conclusive evidence that the creator did create some such, at least at any epoch covered intelligibly by our fossil record. The quiet logic of `yes, but life must have started somewhere' is obliged to recognise that, all the same, we only really know that it happened before the eras of which we have any coherent record.
Humanity has many `origin' stories. Some of these are associated with religions; at least one is associated with science. One of the big areas of struggle between science and religion is the issue of how species come about. Now, for my own part, I was brought up in a culture which believes the story I (arguably unsurprisingly) call scientific: at the same time, I was taught the Christian version of the Jews' origin stories. In each culture, I can see serious variation in how folk interpret their own stories, even when all the folk involved think they are `reading it literally'. That same up-bringing taught me to value this diversity and look within it both for hints at useful wisdom and for the channels by which a useful discourse can be opened up between folk coming at this one world we all share with differing perspectives.
So let me set aside, for a while, the question of `how species arise'. I'll use the phrasing of `God creates another species' and ask those who share my origin stories to read it as though `God' meant `evolutionary pressure'. Let me, in effect, explain why scientists who accepted God as creator found, in the data before them, a challenge to their faith.
What does the fossil record tell us ? In examining this, we have to remember the transience of temporal things: any cities there may have been 30,000 years ago can safely be relied on to have vanished without trace long before the earliest civilisations of which we have any record; likewise, we will find no trace of any creature whose remains will reliably have disintegrated in the time that has elapsed since they lived. Still, much is present: and some consistent threads emerge in it.
The story runs like this: though God creates new species throughout the story we can see, God seems to build any given species by plundering ideas from a prior species and making a few adjustments. In places, where there are several species modelled on some recent prior, it may be hard to distinguish whether there was only one prior to some new species or whether God perhaps combined a few related species and adjusted, rather than taking only one species: but even then the candidate priors share a common prior.
Nature is littered with examples of different species filling similar ecological niches: and in some cases, the same `survival problem' has been solved by species with different history of prior species from which ideas were borrowed. Where this has happened, God has kept to the prior form of a species `recently' resident rather than borrowing from some long-extinct species. Similarly, there are many designs of eye in use: yet one can look at the design of eye used throughout chordata and see an irritating feature which is absent in squids and their ilk.
The retina of the eye is both: the `projection screen' on which the lens, various humours and the iris form an image and; the sensory equipment that reports to the brain what image was formed. View it as the `back' of the eye, with the various humours directly in front of it: in the squid's brain, the nerves which connect the retina to the brain run out of the back of the retina towards the brain - they don't get mixed up in the process of image formation or get in the way of putting as much sensory apparatus in the light as possible. In the chordate eye, however, the nerve cells run out the front of the retina, into the path of the light which forms the image; they then cluster together into a bundle which plunges through the retina in order to come out the back (where they could have been to begin with), leaving a blind spot in the visual field of each eye. At the same time, the nerve cells involved have to be neatly invisible in the relevant humour, which introduces a set of tiresome constraints - which could be avoided !Written by Eddy.