T. rex didn't become a chicken!
There is a persistent misconception that Tyrannosaurus rex evolved into chickens. Another myth is that chickens are the tyrannosaur’s closest living descendants. However, neither of these statements is true, nor are they supported by evolutionary biology.
The fossil record shows no evidence that T. rex or any of its close relatives evolved into chickens. Similarly, there is no fossil or phylogenetic evidence for a direct ancestor–descendant relationship between T. rex and any modern bird. Instead, birds and tyrannosaurs share a much older common ancestor within the theropod clade (Brusatte, 2018).
Modern birds, including chickens, are indeed dinosaurs in a cladistic sense. This relationship is comparable to that of snakes within lizards (squamates). Snakes are highly derived squamates that evolved from within the broader lizard lineage, forming a specialized, limbless branch within it. In this way, what we commonly call lizards represents a broader evolutionary grouping that includes snakes (even though snakes are a distinct and highly derived lineage).
Similarly, birds are a specialized lineage that evolved from within Theropoda. They are therefore technically derived, living dinosaurs. Birds descended from small, feathered maniraptoran theropods, and as a result, Dinosauria includes the birds of today (Brusatte, 2018; Xu et al., 2014).
The evolutionary branch of theropods that ultimately led to birds diverged during the Middle to Late Jurassic, more than 150 million years ago. Tyrannosaurus rex, by contrast, lived tens of millions of years later, near the very end of the Cretaceous Period. It belonged to a separate theropod branch known as Tyrannosauridae.
The tyrannosaurids went completely extinct during the end-Cretaceous mass extinction event. They left no surviving descendant lineages (Holtz, 2004).
Chickens and T. rex therefore share only a distant common ancestor, not a direct ancestor–descendant relationship. Birds did not arise from tyrannosaurs, nor did large Mesozoic dinosaurs simply “become” modern birds. Instead, birds represent one surviving branch of a much larger and diverse dinosaur radiation.
Consequently, Tyrannosaurus rex does not exhibit any special evolutionary closeness to chickens compared to any other bird species; all modern birds are equally related to tyrannosaurs through their shared theropod ancestry. Aside from this theropod relationship, birds are not closely related to other major dinosaur groups such as sauropodomorphs or ornithischians. Their connection to those groups lies deeper in Dinosauria’s early evolutionary history, reflecting shared ancestry rather than close relationship.
Birds are more closely related to maniraptoran theropods, such as dromaeosaurs and troodontids, than to tyrannosaurids. Among living animals, the closest relatives to birds are crocodilians, their fellow surviving archosaurs.
Claims that chickens are the closest living relatives of T. rex misunderstand how evolutionary relationships work. Molecular and protein comparisons showing similarities between birds and tyrannosaurs demonstrate shared theropod ancestry, not direct descent or exceptional genetic closeness (Organ et al., 2008). In evolutionary terms, chickens are not descended from T. rex, but are distant cousins that share a deep common ancestry within Theropoda.
The fossil record shows no evidence that T. rex or any of its close relatives evolved into chickens. Similarly, there is no fossil or phylogenetic evidence for a direct ancestor–descendant relationship between T. rex and any modern bird. Instead, birds and tyrannosaurs share a much older common ancestor within the theropod clade (Brusatte, 2018).
Modern birds, including chickens, are indeed dinosaurs in a cladistic sense. This relationship is comparable to that of snakes within lizards (squamates). Snakes are highly derived squamates that evolved from within the broader lizard lineage, forming a specialized, limbless branch within it. In this way, what we commonly call lizards represents a broader evolutionary grouping that includes snakes (even though snakes are a distinct and highly derived lineage).
Similarly, birds are a specialized lineage that evolved from within Theropoda. They are therefore technically derived, living dinosaurs. Birds descended from small, feathered maniraptoran theropods, and as a result, Dinosauria includes the birds of today (Brusatte, 2018; Xu et al., 2014).
The evolutionary branch of theropods that ultimately led to birds diverged during the Middle to Late Jurassic, more than 150 million years ago. Tyrannosaurus rex, by contrast, lived tens of millions of years later, near the very end of the Cretaceous Period. It belonged to a separate theropod branch known as Tyrannosauridae.
The tyrannosaurids went completely extinct during the end-Cretaceous mass extinction event. They left no surviving descendant lineages (Holtz, 2004).
Chickens and T. rex therefore share only a distant common ancestor, not a direct ancestor–descendant relationship. Birds did not arise from tyrannosaurs, nor did large Mesozoic dinosaurs simply “become” modern birds. Instead, birds represent one surviving branch of a much larger and diverse dinosaur radiation.
Consequently, Tyrannosaurus rex does not exhibit any special evolutionary closeness to chickens compared to any other bird species; all modern birds are equally related to tyrannosaurs through their shared theropod ancestry. Aside from this theropod relationship, birds are not closely related to other major dinosaur groups such as sauropodomorphs or ornithischians. Their connection to those groups lies deeper in Dinosauria’s early evolutionary history, reflecting shared ancestry rather than close relationship.
Birds are more closely related to maniraptoran theropods, such as dromaeosaurs and troodontids, than to tyrannosaurids. Among living animals, the closest relatives to birds are crocodilians, their fellow surviving archosaurs.
Claims that chickens are the closest living relatives of T. rex misunderstand how evolutionary relationships work. Molecular and protein comparisons showing similarities between birds and tyrannosaurs demonstrate shared theropod ancestry, not direct descent or exceptional genetic closeness (Organ et al., 2008). In evolutionary terms, chickens are not descended from T. rex, but are distant cousins that share a deep common ancestry within Theropoda.
References
Brusatte, S. L. (2018). The rise and fall of the dinosaurs: A new history of a lost world. William Morrow.
Holtz, T. R., Jr. (2004). Tyrannosauroidea. In D. B. Weishampel, P. Dodson, & H. Osmólska (Eds.), The Dinosauria (2nd ed., pp. 111–136). University of California Press.
Organ, C. L., Schweitzer, M. H., Zheng, W., Freimark, L. M., Cantley, L. C., & Asara, J. M. (2008). Molecular phylogenetics of mastodon and Tyrannosaurus rex. Science, 320(5875), 499.
Xu, X., You, H., Du, K., & Han, F. (2014). An Archaeopteryx-like theropod from China and the origin of Avialae. Nature, 475(7357), 465–470.
Brusatte, S. L. (2018). The rise and fall of the dinosaurs: A new history of a lost world. William Morrow.
Holtz, T. R., Jr. (2004). Tyrannosauroidea. In D. B. Weishampel, P. Dodson, & H. Osmólska (Eds.), The Dinosauria (2nd ed., pp. 111–136). University of California Press.
Organ, C. L., Schweitzer, M. H., Zheng, W., Freimark, L. M., Cantley, L. C., & Asara, J. M. (2008). Molecular phylogenetics of mastodon and Tyrannosaurus rex. Science, 320(5875), 499.
Xu, X., You, H., Du, K., & Han, F. (2014). An Archaeopteryx-like theropod from China and the origin of Avialae. Nature, 475(7357), 465–470.