I am not a biologist, however, I am curious as to how do evolutionary biologists classify a newly discovered hominid fossil as a separate species. If simply the bone structures are a criteria to classify a hominid specimen as a separate species, how do we know that the different bone structures are not a result of a genetic disorder or a terrible mutation in a human? From what I understand, DNA specimen do not survive that long for consideration. Can anyone provide me with a layman explanation of the process of classification which seemingly doesn't involve genetic data?
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$\begingroup$ As a hint towards the homework aspect - you should look up comparative anatomy and understand what differs in a pathologic (i.e. mutant) vs normal state, and how anatomy might differ between humans and other species. $\endgroup$– bob1Commented Jan 2 at 20:33
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$\begingroup$ @bob1 How are we sure that what we consider anatomy of a different species is not a pathology in a human? $\endgroup$– Dr. user44690Commented Jan 3 at 4:23
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$\begingroup$ Ok then - how do you (as in you personally) tell that someone has dwarfism is human vs non-human? Now, look at the proportions of individual in this state in any animal and extrapolate to other populations and think a little about what we might commonly find as remains? $\endgroup$– bob1Commented Jan 3 at 7:55
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$\begingroup$ @bob1 Or rather how do we distinguish between a human toddler and a dwarf of similar height (or built?) based on purely skeletal remains? From what I understand there seems to be some level of guesswork and arbitrariness in how we decide and define a new hominid species which is kind of bothering me. $\endgroup$– Dr. user44690Commented Jan 3 at 8:18
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$\begingroup$ @bob1 And from what you are suggesting, there seems to be an involvement of extrapolation based on the current diversity of primate anatomy. I was under the impression for a while that there exists theoretical models that allowed biologists to predict the anatomy of our hominid ancestors which are later confirmed or refuted accordingly by discovery of new fossil evidence. But from what I searched on google there is no such methodology under implementation as such. Now I am kind of bothered about this whole thing. $\endgroup$– Dr. user44690Commented Jan 3 at 8:26
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2 Answers
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Hey, quite tricky to correlate but good logic👍
When a fossil is being examined, carbon dating is one of the methods used to examine its age. Also, as time passes, organic components of bones are slowly decomposed and get absorbed in the surrounding (into soil) and the entire mass either collapses under pressure or, inorganic materials from its surroundings replace the empty spaces(rest of the body degrades very quickly). Thus if the fossil is determining exact skeletal structure, then it must have inorganic compounds which resemble to the surroundings. If not(it has inorganic materials that are as same as the inorganic materials of an actual bone), then the fossil is unfit for determining organismal structure, bcuz it had collapsed due to external pressure.
Now coming to your question.... for mutation to take place, there is a certain time period with a certain percentage of accuracy. Which means that for such a change that causes gross phenotypic difference, it requires a certain number of generations and fixed progeny-coincidences. Thus we can compare the age of fossil and the approximate probability of a mutation, which caused phenotypic changes in a normal human to look like the one determined by the fossil. By this we can calculate the probability that whether the fossil is a legit "non sapien hominid" or just a mutated human.
And often these probabilities are are very low. So we finally identify the fossil to be of a certain hominid with a certain level of similarity to the modern day human.
Hope you understood the process.
References:
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/fossil-hominid#:~:text=Most%20early%20fossil%20hominids%20appear,9.10).
https://byjus.com/physics/types-of-fossils/
And for further information on fossilisation and chemical nature of fossils, you can check out this pdf:
https://egyankosh.ac.in/bitstream/123456789/69611/1/Unit-9.pdf
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$\begingroup$ Can you provide any references for this process? $\endgroup$ Commented Jan 5 at 4:28
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$\begingroup$ References? Like you want to know in the form of examples or a deeper explanation of the methods and the process? $\endgroup$ Commented Jan 6 at 5:18
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$\begingroup$ Yes. Any references or lecture notes perhaps will be helpful. $\endgroup$ Commented Jan 6 at 6:10
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$\begingroup$ I've attached the links in another answer... $\endgroup$ Commented Jan 7 at 18:36
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The oldest DNA from Africa is 15000 years old. From Europe it's 120,000 years, and from Siberia they have horse and mammoth DNA from molars that are [700,000 to 1,2 milliom years old][1]. It's feasible that they will find homo erectus and habilis DNA from cold regions.
The more bone and tooth fragments the better. Then a comparative anatomist can study the age and differences of the bone fragments and articulations to see if they have a lot of similarity with a currently known homo species.
If they can find enough differences in bones of a specimen, concensus can agree that it's a new species. It takes quite a lot of bone to decide that. a small shard isn't enough. Half a skull/leg is fairly vague evidence and half a skeleton can be generally convincing. [1]: https://www.science.org/content/article/mammoth-molars-yield-oldest-dna-ever-sequenced#:~:text=1.2%2Dmillion%2Dyear%2Dold,record%2C%20reveals%20new%20mammoth%20lineage&text=A%20genetic%20analysis%20of%20long,the%20oldest%20DNA%20yet%20sequenced.
