Is consciousness immaterial?

If it is true we are all evolved beings from microscopic organisms over millions of years via cell growth and evolution, would this not verify, then, that consciousness is truly material in origin?

Yes, sort of, if one precisely defines "consciousness" and show it ("show it" is a technical scientific term for "lots of evidence") to be a product of material manipulation. But nothing like this has been done, it's currently just speculated on, so no.

That our thinking minds evolved and gradually arose, over the years, from some material and previously mindless thing like a single celled organism?

Well, that's just a theory. We don't know how "mentality" (thought, intelligence, agency and everything else associated with it) arises.

Basically, my concern is this: if science is right that consciousness is just chemicals and nerves in the human brain...

Please note, "science" doesn't know (therefore, "science" is not really right) about "consciousness" being just chemicals.

What we have is scientists and philosophers -- and everybody else -- offering opinions ("opinion": also a technical term, which means "I don't know, but maybe...") beyond their expertise, beyond the realm of evidence, beyond consensus.

Furthermore, some of these opinions share common structure and are categorized (mostly by philosophers) to make it easier to both identify them and to analyze them. This opinion, in particular is called "materialism".

... would a large part of Buddhism be disproved? If consciousness is really just a material reaction, would rebirth be impossible then since the mind would exist as brain functioning and cannot transmigrate?

This is highly speculative over an already polemical subject but hopefully suffice to say that a materialistic notion of rebirth is not so far fetched (it just may involve materials and dynamics one is not so familiar with -- like the mystery electricity once was; or the mystery subatomic physics is).

Basically, I seemed to have run into a block in my practice. I just have fundamental doubts about everything now. Is there a way we can prove the mind is immaterial or experiences the birth/death cycle...and is not just the evolved brain?

I'm sorry to hear that.

Speaking of our personal practice, is it that important to have some random scientist to "prove" rebirth? Scientists did not proved that asking questions here is beneficial, yet we do it, trying it ourselves and seeing if and how it works. The same goes for making an effort to understand our suffering and training to see how it gets eliminated (and how it doesn't).

See, for example, how the Buddha approached Mālunkyāputta (MN 63) who was really anxious with questions to the point he felt he could not go on without the Buddha giving him answers.

In a nutshell:

“And what have I declared? ‘This is suffering’—I have declared. ‘This is the origin of suffering’—I have declared. ‘This is the cessation of suffering’—I have declared. ‘This is the way leading to the cessation of suffering’—I have declared.

“Why have I declared that? Because it is beneficial, it belongs to the fundamentals of the holy life, it leads to disenchantment, to dispassion, to cessation, to peace, to direct knowledge, to enlightenment, to Nibbāna. That is why I have declared it.

Finally, I'm not suggesting that we should throw the question of rebirth to the side, but if our uncertainty regarding it becomes an obstacle, I think this is a problem (that is, a problem in addition to the problem we still face: the problem of being subject to suffering).

First of all, "material" or "immaterial" is the modern philosophical term invented by philosophers; "consciousness" is the term invented by the scientists likewise. They fit not to wrap around the Buddhist teaching, but render confusion. In Surangama Sutra, the Buddha explained how the world came into "existence", he used a metaphor, that the world's existence was like a man who rubbed his eyes so hard, that eventually this caused defected eyesight to see flowers appearing in empty space. The fives senses plus thought/mind are the palette, cardboard and pen with the Self created this great painting (my metaphor). I would boldly assume that Buddha will comment out "consciousness is chemical reaction thus is material". In Surangama, Buddha has described that the eyes, seeing, image are not associated as our scientific understanding understood. For example, a blind man has never lost the "seeing" ability although he cannot see, as in a dark room our eyesight is perfect but we cannot see. Thus seeing is not because of eyes, same as consciousness is not because chemical reaction between the brain and nerves. Aniruddha was able to see with supernatural capacity (third eye) though he lost his eyesight. And these 6 are interchangeable if one has achieved the full mastery. Darwinist evolution is one of the mind "games" the thought created, Surangama summed there are 4 types of birth: egg, embryo, humidity, metamorphosis; with 3 phrases of time, past-present-future: 4x3=12 in total major categories. Pure mind renders the being rising up as celestial, all instinct renders the being sinking down, 7 mind vs 3 instinct is the celestial being, 5 mind vs 5 instinct is man, 3 mind vs 7 instinct is animal; what you eat is making what you are: eating crops from the earth man has his feet locked on the ground... I'm roughly grabbing some of the ideas from Surangama, I believe it has the answer to your question. That's based on my reading the Chinese Classic text Sutra, not sure what it's like the English translation.

Buddha never seemed to accept materialism, or that mind was material, in much of the scripture.

A being has a material structure and mental structure (Namarupa).

If it is true we are all evolved beings from microscopic organisms over millions of years via cell growth and evolution, would this not verify, then, that consciousness is truly material in origin? That our thinking minds evolved and gradually arose, over the years, from some material and previously mindless thing like a single celled organism?

There are fine material and immaterial planes where thinking is not linked to a material organ like the brain. Thinking is linked heart in the Abhidhamma. In immaterial spheres since there is no materiality thinking cannot be material base.

As for organism in this world, Buddhism does not seems to contradict Darwinism but Buddhism evolution is much more, where the evolution began from celestial beings.

Their body was still coarse and roughly shaped. Thus, after a very long time, the mud-like substance began to be exhausted. Then, mushroom-like plants began to grow so fast that they replaced the mud-like ocean. The creatures began to devour them as well, and they also found it delicious, like sweet honey and milk. Their body hardened more and details began to turn finer.

After another very long time, the mushrooms also began to be exhausted, replaced by cassava or turnip-like plants. They also began to devour them night and day, and thus they began to notice differences amongst them. As the changes of their bodies varied between each other, the concept of difference arose. The concepts of the beautiful and the ugly were born. The beautiful scorns the ugly and they became arrogant because of their appearance.

Then, after the turnips, the earth was grown with rice plants. ...

Source: Aggañña Sutta

Basically, my concern is this: if science is right that consciousness is just chemicals and nerves in the human brain, would a large part of Buddhism be disproved? If consciousness is really just a material reaction, would rebirth be impossible then since the mind would exist as brain functioning and cannot transmigrate?

There is both a physical and non physical part. Science cannot proble not non physical aspects hence. So any scientific proof and disproof is not considering all the facts.

Basically, I seemed to have run into a block in my practice. I just have fundamental doubts about everything now. Is there a way we can prove the mind is immaterial or experiences the birth/death cycle...and is not just the evolved brain?

This you have to mediate. If you develop any higher knowledges then it is possible, but only a few are capable of this.

Words such as 'material' & 'immaterial' are just translations. It is probably best to not impute our own ideas onto these translations, particularly the idea that 'materiality' & 'immateriality' are independent of eachother.

For example, meditators can enter into the 'immaterial jhanas' where the physical body cannot be felt anymore. However, the mind of the meditator is still dependent upon its physical body.

Regarding the word 'material' or 'rupa', this is pointing to the nature of physical things, such as the physical body. For example, about 'rupa', the Pali scriptures have certain descriptions of 'rupa' that do not pertain to 'consciousness', such as:

And why do you call it 'form' (rupa)? Because it is afflicted (ruppati), thus it is called 'form.' Afflicted with what? With cold & heat & hunger & thirst, with the touch of flies, mosquitoes, wind, sun & reptiles. Because it is afflicted, it is called form. (Khajjaniya Sutta.)

...the body — endowed with form, composed of the four primary elements, born from mother & father, nourished with rice & porridge, subject to inconstancy, rubbing, pressing, dissolution & dispersion — is eaten by crows, vultures, hawks, dogs, hyenas or all sorts of creatures... (Mahanama Sutta.)

The four great elements, bhikkhu, are the cause and condition for the manifestation of the form aggregate. (Puṇṇamā Sutta.)

As for consciousness, it is defined as follows:

And why do you call it 'consciousness'? Because it cognizes, thus it is called consciousness. What does it cognize? It cognizes what is sour, bitter, pungent, sweet, alkaline, non-alkaline, salty & unsalty. Because it cognizes, it is called consciousness. (Khajjaniya Sutta.)

The Pali suttas state in many places that there can be no arising of consciousness independent of a physical body &/or sense organs (e.g. MN 38, SN 22.53, SN 12.67, SN 22.82, etc).

The Pali scriptures do not teach consciousness transmigrates from life to life. Ideas of re-linking or a stream of consciousness were created by later-day Buddhist philosophers & scholars. In MN 38, the bhikkhu Sati was severely reprimanded, admonished & censured by the Lord Buddha for holding such a pernicious viewpoint that consciousness transmigrates.

In conclusion, if science is right that consciousness arises from chemicals and nerves in the human brain, this will not disprove what the Buddha taught but, to the contrary, will affirm what the Buddha taught about consciousness.

However, it will disprove what the majority of Buddhists believe about Buddhism.

Note: Although consciousness may arise from chemicals & nerves, chemicals & nerves are not the operation of consciousness. Chemicals & nerves are 'rupa' where as 'consciousness' is 'knowing' or 'cognition'. Thus, consciousness is not chemicals & nerves, which is why consciousness is immaterial. The immaterial (nama) arises from the material (rupa).

Mind-and-body (nama-rupa) is the cause and condition for the manifestation of the consciousness aggregate. SN 22.82

What Buddha taught is not the science which layman(non-Ariya) call at present. Science cannot prove everything Buddha taught so far, maybe later. Sansara and the end of the samsara(i.e. Nibbanna) many people tried to find out but in vain. What Buddha said is one have suffering, there is cause of suffering and liberation of suffering. To know this Buddha mentioned something related to mind and matter what most people disputed. Just like making dispute on the footprint of the person whom one want to trace although that same person is just in front of one. One has already been injured by the spear which should be removed instantly so that one can be saved in time. If one practice insight meditation towards Nibbanna and attaining Bhanga Nana(knowledge of dissolution) no more doubts on these things. If you observe so-called empirical science, particle theory, wave theory , dual theory of light even Einstein find difficulty in understanding, will be clear even by lower Nana of insight meditation. Then why some who attain Nibanna never mention is , it is NOT WORTH MENTIONING. Let alone Nibanna which is the ultimate goal. So what I suggest you is practice meditation if you believe it or not, once you attain these levels mentioned, all doubts will be cancelled.