Birds and mammals are two distinct groups of animals that evolved from different ancestors. However, there are some similarities between the two groups that point to a distant evolutionary relationship.
Classifying birds and mammals
Birds and mammals are both vertebrates, meaning they have backbones. More specifically, birds are classified in the group Aves while mammals belong to the class Mammalia. These are two distinct and separate classes of animals.
There are key differences between birds and mammals:
- Birds are warm-blooded animals that lay eggs. Mammals are warm-blooded but give live birth to their young.
- Birds have feathers and wings; mammals have fur and do not have wings.
- Birds have lightweight, hollow bones; mammals have dense bones.
- Birds have a beak with no teeth; mammals have teeth.
Based on these obvious differences, it may seem that birds and mammals are completely unrelated. However, the two groups share some important similarities that suggest a common distant ancestor.
Evidence of a common ancestor
Birds and mammals share a handful of features that provide evidence of a common ancestor:
- They both have a four-chambered heart.
- They are both endothermic, meaning they can regulate their own body temperature.
- They both have a large and developed brain.
- They both have a diaphragm, a muscle that assists breathing.
- They both have a specialized ear structure with three small bones that transmit sound.
These shared characteristics indicate that birds and mammals descended from a reptilian ancestor that lived approximately 300 million years ago. This ancestor is believed to have had a four-chambered heart and endothermic abilities. Over millions of years of evolution, this ancestor’s lineage diverged into synapsids, which eventually gave rise to mammals, and sauropsids, which gave rise to birds.
The reptilian ancestor
The reptile group from which mammals and birds evolved is called synapsids. The earliest synapsids appeared on Earth around 320 million years ago during the Carboniferous period. They looked like lizards and were small, about the size of a rat.
Around 300 to 280 million years ago, synapsids gave rise to two major groups:
- Sauropsids, which includes modern reptiles as well as dinosaurs and birds.
- Therapsids, also called mammal-like reptiles, which eventually gave rise to mammals about 200 million years ago.
This divergence is evidence that synapsids contained features that were passed down and modified through evolution to create both mammals and birds. Sauropsids retained many reptilian features but evolved into dinosaurs and later into modern birds. Therapsids became increasingly mammal-like, giving rising to mammals while losing their reptilian features over time.
Mammal-like features in therapsids
Therapsids, the ancestors of mammals, evolved several mammalian features over millions of years:
- Legs positioned underneath the body rather than sprawled out to the sides
- Larger brains with expanded regions associated with memory and learning
- Specialized teeth for chewing
- A secondary palate allowing them to eat and breathe at the same time
- Whiskers and fur
- Warm-blooded metabolism
- Expanded role of the cerebellum, allowing for coordination and balance
- Improved hearing
These adaptations allowed therapsids to diversify and flourish, eventually giving rising to the earliest mammals about 200 million years ago. The mammal descendants continued to evolve and acquire mammal characteristics.
When did birds and mammals diverge?
Birds and mammals diverged from their common ancestor approximately 300 million years ago during the late Carboniferous period. The diagram below outlines the evolutionary timeline:
320 million years ago – The earliest synapsids evolved with both reptilian and mammalian features.
300 million years ago – Synapsids diverged into sauropsids and therapsids.
200 million years ago – Mammals evolved from therapsids.
150 million years ago – Birds evolved from feathered dinosaurs.
Image credit: Courtesy of Wikipedia
In summary, both birds and mammals evolved from synapsids. But while mammals came from therapsids, birds descended from sauropsids, specifically feathered dinosaurs. Although they evolved separately for millions of years, the evidence of their common ancestor remains in shared features like endothermy and a four-chambered heart.
Similarities and differences between birds and mammals
Despite their evolutionary divergence, birds and mammals retained some key similarities. However, they are still anatomically distinct groups. This table summarizes some of the major similarities and differences:
Feature | Birds | Mammals |
---|---|---|
Skeletal system | Lightweight, hollow bones | Dense bones |
Heart | 4 chambered | 4 chambered |
Metabolism | Endothermic | Endothermic |
Breathing | Uni-directional flow through lungs | Bi-directional flow through lungs |
Young | Lay eggs | Give live birth |
Covering | Feathers | Hair/fur |
Teeth | Beaks, no teeth | Teeth present |
While birds and mammals have skeletal, cardiovascular, and metabolic similarities, they have adapted those features differently to suit their respective lifestyles and environments. Birds evolved for flight with lightweight bones, feathers, and high metabolic rates. Mammals evolved for diverse habitats, giving birth to live young and developing fur for temperature regulation.
Similarity in early development
Birds and mammals share another intriguing similarity in how their bodies develop as embryos. All vertebrate embryos develop pharyngeal (throat) arches, bulbous structures that give rise to structures in the head and neck. Fish and reptiles develop six arches, but birds and mammals develop only five.
This suggests that the common ancestor of birds and mammals had already lost one pharyngeal arch during its evolution from fish-like creatures. That early developmental similarity was passed down and preserved in both groups, even as they diverged onto land and into the air.
Genetic similarities
Modern genomic studies have allowed scientists to directly compare the genes of birds and mammals. Overall, birds and mammals shared approximately 70% of genes. Bats, primates, and rodents shared over 90% of their genes with humans.
Notably, birds lack a gene called Bmp16 that is present in all mammals. In mammals, Bmp16 plays a role in the development of the mammary glands and feathers. The absence of this gene in birds supports their evolutionary divergence. However, other genes involved in limb development, hearing, vision, and metabolic functions are conserved between birds and mammals.
Conclusion
In summary, birds and mammals evolved from a common amniote ancestor approximately 300 million years ago. They retain similarities in their four-chambered hearts, endothermy, and brain development that provide evidence of this shared ancestry.
However, millions of years of evolution adapted those features to the unique lifestyles of birds that fly and mammals that give live birth. While similarities remain due to their common origin, birds and mammals have become anatomically distinct based on their specialized adaptations over time.
Understanding the evolutionary relationship between birds and mammals provides insight into how complex structures like the heart, lungs, and brain evolved in different groups of animals. Tracing their shared history deepens our understanding of how present day biodiversity developed from ancient species.