Something I've been thinking about, after seeing enough "older than we thought" pieces, is how our incomplete fossil record has an inherent asymmetry in it - new discoveries can never give us a later date for when something appeared, but they can give us an older date, so our "present" understanding of the past has an inherent bias towards assigning too late of dates for things. I don't know what implications this has for the nuances of our understanding of the history of our planet (I'm sure scientists are aware of this and generally take this into account), but it's kind of intriguing.
> new discoveries can never give us a later date for when something appeared
It's probably not as common as the other way, but it's certainly possible. A new discovery can cause the estimated time of an event to be revised more recent, moving it closer to the present.
Cool. I guess it depends on the uncertainty level of the previous consensus. Also whether we're talking about a change in the estimated date of a one-time event, or a lengthening of the duration that something existed.
You're right about this. In practice, geologists are very used to providing asymmetrical or one-sided bounds on many estimates (not only time estimates; also estimates of quantities like the thickness of a rock layer before the top was eroded off and lost to history), and these are often linked to another event so they are sort of relative estimates.
We often say that some piece of evidence establishes the minimum age, or the maximum age, of a geologic event. Then we try to find another event that provides the other constraint. For example, let's say we want to know when a specific meteor hit the earth. The rock that are impacted by the meteor are 100 million years, so we know that the meteor impact was younger than 100 million years. Then if we were able to find a rock unit deposited on top of the impact crater and impacted rocks, that is not itself damaged, we can infer that the undamaged rock unit is younger than the impact. If we date it at 80 million years old, we know that the meteor impact occurred between 100 and 80 million years. (We refer to these relationships as 'cross-cutting relationships' because visually, a younger feature may 'cut across' an older feature and we find and describe these relationships to develop the geologic histories.)
In fact, the entire geologic time scale as it's commonly known (using time categories like the Paleozoic, Jurassic, Cretaceous, etc.) was basically set in complete relative terms without knowing the absolute dates. Then, when absolute dating based on radioactive decay of elements in earth materials became common in the 20th century we started to pin down the whole framework.
I think that there is a really interesting logical structure to the whole enterprise that is begging to be modeled using tools that are more familiar to software engineers. For example I think that a lot of the relationships could be represented abstractly and analyzed using graph theory. But I have not had time to make much progress with this as geology has so many little exceptions that formalizing it is a long and tedious process.
>> new discoveries can never give us a later date for when something appeared, but they can give us an older date
But neither are useful. The fact that a structure appeared at time X is not an indication that it continued until time Y. Evolution is not linear at smaller scales. Structures evolve/appear only to disappear through the same processes that created them. The fact that this critter had a particular shape of eye is not an indication that it passed that structure to later lifeforms. It is certainly possible, but the concept that something happened "first" is irrelevant in a world where near-identical structures regularly evolve independently.
In practice, paleontologists look at the passage of traits down evolutionary lineages regularly. You are correct in stating that the same features can develop independently, but that does not mean they do every time. I don't think the assertion that 'evolution is not linear at smaller scales' holds up to much scrutiny. Life as we know it results from the passage of traits encoded in genes, otherwise there would be no evolution; we would have basic unicellular life spontaneously arising from the primordial ooze over and over again Groundhog Day style.
> The fact that this critter had a particular shape of eye is not an indication that it passed that structure to later lifeforms.
In the parlance of the geosciences, depending on the context, we'd call this a suggestion rather than an indication, as there may be weaker evidence or credible alternative hypotheses. However depending on the scales of the time and speciation differences between the species under consideration it may be very strong evidence.
For example, it's highly unlikely that the presence of limbs in both Homo erectus and Homo sapiens is coincidental and Homo sapiens have limbs that evolved independently of Homo sapiens. This is very different than asserting that wings in both bats and birds are indications that one evolved from the other.
Thats right, an evolutionary advantage from birth defect does not 100% result in a successful lineage. It can just die and not even pass it on, then later the same defect can appear again and again until if results in a lineage.
Its an advantage to see better, but it does not guarantee survival just a slightly higher percentage (which is enough to thrive)
compound eyes consist of thousands of ommatidia which are the individual eye component ... these animals determine direction of movement of their prey in the layer of neurons directly attached to each of these ommatidia ... each ommatidia has 6 immediate neighbor ommatidia ... leaving a given ommatidia are 6 optic neurons connecting that ommatidia with each of its neighbor ommatidia ... when a prey moves across the field of view directly in front of a given ommatidia a signal spike is transmitted in all directions outward through these 6 outgoing optic neurons ... interesting aspect is these neurons incur an intentional time delay in this spike signal ... when the time it takes a prey to move from being in front a given ommatidia to being in front of a neighbor ommatidia matches this optic neuron time delay the neighbor ommatidia senses this as a doubly large spike signal ... this gives the animal information as to the direction of prey movement across its visual field ... this optical computation directly is this simple layer of neurons gives the compound eye animal extremely fast response time when hunting down a moving prey ... think of watching a dragon fly chase down a flying mosquito
Ok I see the "fly's eye" texture. But it's a leap to suggest this means, the eye was built the same as a modern insect. It could have been made of external light-sensitive cells instead of internal; it could have been a pressure sensor; it could even have been a touch sensor!
I'd want a lot more evidence over millions of years, showing continuous use of this structure in sea creatures that lead to the modern insect form. Otherwise, just another 'his-story'?
I don't think your skepticism is warranted. EamonnMR provides a link to an article describing the paper Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes [1]. This animal was a pre-cursor to modern crustaceans like shrimp and crabs. The compound lens in this fossil has about 16K lenses.
Today's OP describes the paper Insights into a 429-million-year-old compound eye [2] in a trilobite fossil with about 200 lenses.
Logically, I can't imagine how free-swimming predators could have evolved in the Cambrian without eyes. Having two very distinct species of arthropods with fossilized compound eyes is not "his-story" but key evidence that furthers our understanding of the Evolution of the Eye [3].
The particular structure of the trilobite eye didn't become the insect eye; it's just an example of a more general structure. Trilobites had calcite (mineral) lenses; insects don't. And they were lenses - proper biconvex lenses - not push-buttons.
