Edited for a less broad question, I've retained my original question in the event anyone wants to try to answer it

Narrowed down question

According to evolution we should all share common ancestors, straight to the very first cell. I am trying to find evidence for these ancestors. So therefore as a start, what fossil evidence do we have for the first official mammal to roam the earth? What was the defining moment and change from whatever it was before to a mammal?

Original question

I'm just doing some research for my biology class and I was wondering what evidence we have for the elusive "change of kinds" question? And for clarification, I define kinds as the branches of life (essentially birds, reptiles, mammals, amphibians, fish, etc etc). Do we have evidence for the common ancestors that link the vastly different species we have? Because obviously a human, tortoise, jellyfish, and say tarantula are vastly different, each their own distinct "kind", but according to evolution there should be common ancestors and many distinct species leading up to them. We all came from one thing. Man and primates came from one thing. Mammals came from one thing. Where do we get that distinct change of a kind, from say a fish to mammal?

There should theoretically be tens of thousands of in between links and species that were what led to everything today, correct? Therefore, we should have mountains of fossil evidence to support it, yet from what I've researched I've found very lacking evidence and generally just see posts insulting creationists and skimping the question. So by asking the question myself hopefully I'll get a legitimate and clear answer that is the truth.

  • $\begingroup$ Yes there is tons of evidence, including whole living groups of species as well as fossil missing links. Every time science finds one of these "missing links", though, creationists pop in and say "oh well what about the link between that and (other group)" - it's really quite frustrating. Try an academic source on evolution like evolution.berkeley.edu/evolibrary/home.php $\endgroup$
    – Bryan Krause
    Feb 8, 2018 at 17:50
  • $\begingroup$ We indeed have thousands (in fact, probably hundreds of thousands) of evidence. Note that fossil are not the only source of evidence for such common ancestry. We also have genetic data, phenotypic, biogenographical and geological data that all agree on the same general story. The question is very broad though and it is hard to know exactly what study you would like to hear from. You might want to have a look at this post who list to thousands of studies explained in lay terms. $\endgroup$
    – Remi.b
    Feb 8, 2018 at 18:00
  • $\begingroup$ For a short and easy introduction to evolutionary biology, you might be interested in having a look at Evo101 by UC Berkeley $\endgroup$
    – Remi.b
    Feb 8, 2018 at 18:01
  • 1
    $\begingroup$ @Timble Great! That's a question sufficiently well defined to be answered. If you can edit your post so that this becomes your question, we will happily attempt to find evidence of fossils of early mammals. $\endgroup$
    – Remi.b
    Feb 8, 2018 at 18:12
  • 3
    $\begingroup$ Part of the problem is that defining the exact point at which a "change in kind" took place, or even whether it did take place, is essentially a human construct, while evolution is continuous. Take for instance the people who say that birds are dinosaurs, denying that a "change in kind" took place. Or even that all dinosaurs were one kind, rather than (at least) two distinct sorts (saurischians & ornithischians, IIRC). $\endgroup$
    – jamesqf
    Feb 8, 2018 at 19:40

2 Answers 2


Obviously, it's impossible to point to "the" earliest mammal; dividing lines are pretty much arbitrary. However, a reasonable representative of a very early mammal or very late mammaliaform is Arboroharamiya jenkinsi, described in A new arboreal haramiyid shows the diversity of crown mammals in the Jurassic period, from roughly 155-160 million years ago. That shows a number of fossils, including this quite nice one: enter image description here

The supplementary information shows some cranial features, including this: enter image description here

A related species from roughly the same time period, Haramiyavia clemmenseni, had CT scanning done on this fossil mandible:

enter image description here

yielding images like this:

enter image description here

Similarly, the euharamiyidans were either very early mammals or very mammal-like cynodonts, which are known from a number of quite nice fossils including these three (described in Three new Jurassic euharamiyidan species reinforce early divergence of mammals):

enter image description here

  • $\begingroup$ Good answer +1. It would be nice to have an estimate of how old these fossils are. $\endgroup$
    – Remi.b
    Feb 8, 2018 at 21:56

It is a mistake to think that there was ever a "first" mammal. There were only animals that were more mammal-like or less mammal-like. To ask what was the first mammal is like asking who was the first tall person. What is "tall"? "Mammal-ness" is on a continuous spectrum just as "tallness" is. All we can do is acknowledge transitional forms like those mentioned by @iayork.


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