Kratom, which has the scientific name Mitragyna Speciosa, is actually closely related to Coffee. Indeed, Mitragyna Speciosa and the Coffea Genus of flowering plants, which are the source of Coffee beans, are both in the biological Family Rubiaceae, as well as several other medicinal plants.
A biological Family is one of the most specific groupings in the biological taxonomic system. There are actually 6 groupings above Family, including Life, Domain, Kingdom, Phylum, Class, and Order. A Family is a set of Genera, the plural word for Genus, which are closely related to each other. The only taxonomic levels which are more specific than Family are Genus, which is a set of species that are very similar, and then the Species itself.
Another way to think of it is that a Biological Family shares a common ancestor and has similar characteristics. In this specific case, the Rubiaceae Family is defined by having opposite leaves that are simple and entire, interpetiolar stipules, as well as several other physical characteristics like tubular sympetalous actinomorphic corollas, and an inferior ovary.
In simpler terms, all Species in the Rubiaceae Family share a set of common physical characteristics. Therefore, Coffee and Kratom share common physical characteristics and have a common ancestor.
This actually makes a lot of sense when considering that Coffee is the most popular plant for work, since it enhances energy and focus while getting rid of fatigue. Likewise, Kratom also enhances energy and focus while getting rid of fatigue.
Basically, not only do Kratom and Coffee share the same Biological Family and therefore the plants they are sourced from have similar physical characteristics, the psychoactive effects of Kratom and Coffee also share similarities.
That being said, Kratom can be even more effective than Coffee since it is simultaneously quite relaxing and euphoric, while also having powerful painkilling capabilities. Indeed, in the region that Kratom natively grows it has been used for hundreds or thousands of years as a work enhancer, perhaps moreso than Coffee.
It is also interesting to note that there are several other plants with medicinal properties in the Rubiaceae Family. The bark of Cinchona trees contains a variety of alkaloids including Quinine, which is effective for treating Malaria as well as restless leg syndrome. Woodruff contains Coumarin, a natural precursor of the blood thinning drug Warfarin. Psychotria Viridis contains Dimethyltryptamine (DMT) and can be made into Ayahuasca, a powerful psychedelic. The bark of Breonadia is commonly used in African traditional medicine to fight diarrhea and stomach/digestive problems, and has anti-bacterial and anti-fungal properties, in addition to being a treatment for arthritis.
Thus, Kratom, Coffee, and several other useful medicinal plants including Cinchona, Woodruff, Psychotria Viridis, and Breonadia are in the Rubiaceae Family, and therefore are closely related since they share a common set of physical characteristics and have a common ancestor.