Now that I have completed the introductory and two body paragraphs, I will go on and use the final body paragraph to illustrate the introductory adverbials that take a comma. I will follow my usual structure: First I introduce the paragraph thesis. Second I state my paragraph thesis. Next I will support my thesis and give reasons. Finally I will draw my conclusions. There will be no transition signal as this is my final body paragraph. After that I will only write a full concluding paragraph.

Many animals are not so different from human beings. We are genetically more similar to monkeys or pigs than to spiders and shrimps. Yet we call all of them animals. As there are plenty of biological and mental similarities, we should grant to them what our ethics tells us to grant to our fellow human beings: to do as you want to be done to. Animals can feel pain. Comstock (2015), for example, explains that ‘the amygdala, the cerebellum, the thalamus, which are all involved in processing pain in us, are all found in cattle. We can therefore conclude that the pain animals feel is highly similar to ours and, to put it the other way round, we should not do as we do not want to be done to. When we apply this rule to humans, we usually consider that not only pain matters but also happiness. We grant every human being to pursue their happiness. Animals can also feel pleasure and anticipate a positive future. Jeff MacMahan (2015), professor at Oxford, quotes an experiment by Broom that suggests strongly that animals – cattle in this experiment - not only anticipated pleasure when learning how obtain food but also seemed to be aware of and pleased about their own accomplishment.¹ Of course, they showed this rather by jumping, kicking and galloping. When we kill animals, we end the pursuit of happiness of beings of another species. Something we would not like to be done to ourselves; something we should not do to a member of another species either.
^{1 Gary Comstock is professor of philosophy at North Carolina State University. Abridged and adapted from: https://www.bbc.com/news/science-environment-34541077, 2015.}

The following introductory adverbials take a comma because they function like a comment on the whole sentence and are thus similar to additional information. When they are used in the middle of the sentence, they take two commas.

1 Introductory personal stance adverbials
According to Smith (1999), …
On this view, … Surprisingly,
Significantly, …
Obviously,…
Clearly, …
In fact, …
 
2 Adversative transitionals
However,
Nevertheless,
On the contrary,
Rather,
Hence,
What is more,
Yet*
Therefore**

* exception – optional, hardly ever takes a comma because it is so short
** exception - hardly ever takes a comma

3 Summative transitionals
All in all, …
In conclusion, …
Altogether, …
As has been said, …
Finally, …
In brief, …
 
4 Additive transitionals
Moreover,
Furthermore,
In addition,
Again,
Besides,
 
5 Causal transitionals
Consequently*,
Because of this,
For this reason,

* can not follow a comma

There is yet another problem with transitionals. It stems from interference with German grammar. In German grammar the equivalents of the adverbs ‘however‘, ‘therefore‘, ‘hence‘, ‘consequently‘, ‘nevertheless‘ can be used after a joining comma. In English grammar they can't.

Following a plant-based diet was long thought to mean giving up your favorite meat dishes. However, there are many plant foods that are delicious meat substitutes.
Two members of the expedition were too ill to continue. Nevertheless, the others decided to press on.
Legumes, for example, are one of the healthiest foods on the planet. Therefore we should substitute meat by them whenever possible.
You can find recipes that turn lentils, chickpeas and other legumes into burgers, ground meat and meatloaf. Consequently, minimally processed soy foods like tofu are another great substitute.
Many vegetables can also sub in for meat. Hence, jackfruit, mushrooms, eggplant and potatoes make perfect meat substitutes.