It has its own specific “how,” which prescribes that we must understand existence’s specific being unto its world as concern for that world. This is not just any characteristic of existence, but one that is determined by existence’s basic way of being. Only with reference to this basic way of being can we phenomenally understand a-priori-being-with in a broad enough way to bring out its ur-temporal structure.
Some might think that when we define existence in terms of the structure of being-in-the-world, we are grounding our interpretation of existence in some general biological structure.  This characteristic of being-in-the-world, they might say, pertains in a certain sense also to plants and animals, because to the degree that they are at all, they have their worlds, their own specific (broad or narrow) environments. This ontological determination “being-in-the-world,” if understood within that horizon and applied to existence (the being of human beings) is, they will say, merely a species of the broader genus called “to have a world.” It is a short step to understanding things that way. But on closer view it is clear that whereas we may have to attribute “having a world” to plants and animals, we can do so only insofar as we have first understood this structure as it pertains to our own existence as such.
We can arrive at the biological basis of human being—i.e., the basic structure of our “biological being” in the narrow sense—only if beforehand we have already understood “biological being” as a structure of existence. It does not work in reverse. We cannot derive the determination “being-in-the-world” from biology. It must be acquired philosophically. This means that even biology qua biology cannot see the structures of “biological being” in its specific objects, for qua biology it already presupposes such structures when it speaks of plants and animals. Biology can establish and determine these structures only by transgressing its own limits and becoming philosophy. And in fact more than once in the course of the development of modern biology, especially in the nineteenth century (although only in very general characterizations and vague concepts), biologists have referred to this structure and to the fact that animals above all, and plants in a certain sense, have a world. To my knowledge the first person to have run across these matters again (Aristotle had already seen them) was the biologist K. E. von Baer,10 who referred to these structures in his various lectures, but only in passing, never really thematically. More recently his suggestions have been taken up by von Uexküll,11 who now deals with this problem thematically,
10. [The Prussian-Estonian Karl Ernst von Baer (1792–1876), who, as the founder of comparative embryology, discovered the mammalian ovum and the notochord. Heidegger mentions him at SZ, p. 78 / tr. 84.]
11. [Jacob von Uexküll (1864–1944), best known for his Umwelt und Innenwelt der Tiere (Berlin: Springer, 1909; 2d exp. edition, 1921), as well as for his Theoretische Biologie (Berlin: Paetel, 1920), which was translated by Doris L. Mackinnon as Theoretical Biology (London: K. Paul, Trench, Trubner / New York: Harcourt, Brace, 1926).]