Birds are a diverse group of vertebrates that are found worldwide. There are over 10,000 recognized living species of birds that inhabit ecosystems across the globe. But what are the closest living relatives of birds from an evolutionary perspective? Understanding the phylogeny or evolutionary relationships between different groups of organisms can provide important insights into their biology, morphology, behavior and more.
Dinosaurs
One of the closest living relatives of birds are dinosaurs. Birds are considered the only surviving lineage of dinosaurs. Evidence shows that birds evolved from small feathered theropod dinosaurs during the Late Jurassic period over 150 million years ago. Features that both birds and dinosaurs share include:
- Feathers
- Hard-shelled eggs
- Fast growth rates
- Hollow bones
- Wishbone
- Three-toed feet
Fossil evidence demonstrates that dinosaurs like Velociraptor and Microraptor already had wings and feathers very similar to modern birds. While the other dinosaur lineages went extinct around 66 million years ago, the evolutionary branch that led to birds survived. Birds today still retain many anatomical, physiological and behavioral traces of their dinosaur ancestors.
Crocodilians
Crocodilians, including crocodiles, alligators, caimans and gharials, are the closest living relatives of birds after dinosaurs. Crocodilians and birds form a group called Archosaurs together with extinct dinosaurs and pterosaurs. Archosaurs originated during the Triassic period over 245 million years ago. Birds and crocodilians share these archosaur characteristics:
- Four chambered hearts
- Closed circulation
- Respiratory systems with unidirectional airflow
- Long snouts
- Thin walled eggshells
- Parental care of eggs and young
Genetic studies estimate that crocodilians and birds diverged around 255 million years ago. But despite this evolutionary separation, crocodilians remain the closest living cousins of birds today.
Turtles
Turtles are another group of reptiles that are closely related to birds. Turtles, birds and crocodilians all belong to the clade Archelosauria. Archelosaurs originated around 260 million years ago during the Permian period. Turtles share several anatomical and physiological traits with birds including:
- Toothless beaks
- Protective scales
- Limited mobility between shell elements
- Large eggs with calcium carbonate shells
- Slow growth and maturation
Molecular studies indicate that turtles diverged from archelosaur ancestors around 255 million years ago, around the same time as crocodilians. This makes turtles another one of the closest living relatives to today’s birds.
Lizards and Snakes
Lizards, snakes and a group called Rhyncocephalia (tuataras) together form the reptile clade Lepidosauria. Lepidosaurs split from archosaurs like birds around 270 million years ago during the Permian. Birds retain some ancestral anatomical traits with lepidosaurs including:
- Amniotic eggs
- Scaly skin
- Double-penis (hemipenes)
- Slow metabolic rates
While farther removed evolutionarily from birds than crocodilians and turtles, snakes and lizards still represent an early branch of reptiles that is closely related to birds. As their ancestors diverged over 250 million years ago, many unique adaptations in each lineage evolved since then.
Mammals
Mammals are more distantly related to birds than other reptiles. Mammals and reptiles including birds split over 320 million years ago during the Carboniferous period. Some anatomical and physiological similarities birds still share with mammals include:
- Endothermy (warm blooded metabolism)
- Lungs
- Complex brains
- High activity levels
- Parental care of young
These traits were likely present in common amniote ancestors but independently evolved to higher levels in the bird and mammal lineages. While not the closest relatives, mammals still retain some deep evolutionary connections with birds due to their shared evolutionary origins.
Amphibians
Amphibians like frogs, toads, salamanders and caecilians are more distantly related to birds than reptiles or mammals. Amphibians diverged from amniote ancestors around 340 million years ago during the early Carboniferous. A few similarities birds still share with amphibians include:
- Tetrapod body plan
- Bone marrow
- Some amphibians have hard egg shells
But major differences have emerged over hundreds of millions of years of evolution. For example, amphibians never evolved amniotic eggs, water-tight skin, or the ability to live far from water sources. They represent an early branch of tetrapods distinct from birds’ later theropod dinosaur and archosaur ancestors.
Fish
As tetrapods, birds share a very distant ancestor with fish that lived over 375 million years ago in the Late Devonian. A few traits birds still share with fish include:
- Gills in embryonic development
- Paired appendages
- Lightweight, rigid proteinaceous feathers/scales
But fish are a very distant relative to birds. Key adaptations such as living on land, lungs, amniotic eggs and limbs for flying set birds’ archaeosaur ancestors on a separate evolutionary path from fish over 350 million years ago.
Summary Phylogeny
Here is a summary diagram of the phylogenetic relationships between birds and other vertebrate groups:
Group | Divergence from Birds | Shared Characteristics |
---|---|---|
Dinosaurs | 150 million years ago | Feathers, hollow bones, wishbone |
Crocodilians | 255 million years ago | Four chambered heart, parental care |
Turtles | 255 million years ago | Toothless beak, calcium carbonate eggshell |
Lizards/Snakes | 270 million years ago | Scaly skin, amniotic eggs |
Mammals | 320 million years ago | Endothermy, complex brains |
Amphibians | 340 million years ago | Tetrapod body plan, bone marrow |
Fish | 375 million years ago | Paired appendages, gills as embryos |
This table summarizes the evolutionary divergence times and shared characteristics between birds and other major vertebrate groups. Dinosaurs share the most recent common ancestry with birds, while fish represent the most distant relatives on this list.
Conclusion
In summary, from a phylogenetic perspective, birds are most closely related to reptiles including dinosaurs, crocodilians, turtles, lizards and snakes. Mammals also share connections with birds but are more distant relatives. And fish represent the most distant relatives among vertebrates. Understanding these evolutionary relationships provides insights into the origins of birds’ unique biology and characteristics.