Evidence for Evolution
3. Embryology
Learning outcomes
- I can describe similarities among embryos of different species.
- I can explain how embryological evidence supports evolution.
- I can compare stages of embryonic development.
- I can identify shared developmental features.
- I can relate embryology to common ancestry.
What Is Embryology?
Embryology is the study of how organisms develop from fertilization through their early stages of life.
An embryo is an organism during an early stage of development.
Scientists can compare the embryos of different species to investigate:
- Similarities in development.
- Differences in development.
- Shared anatomical structures.
- Developmental patterns.
- Evolutionary relationships.
Similar developmental patterns among different species can provide evidence that those species inherited aspects of their developmental programs from common ancestors.
Embryonic Development
In sexually reproducing animals, development usually begins when a sperm cell fertilizes an egg cell.
This produces a zygote.
The general sequence is:
Fertilization → zygote → cell division → embryo → further development → fetus or juvenile stage
The exact terminology and developmental stages vary among groups of organisms.
During development, cells:
- Divide.
- Move.
- Change shape.
- Differentiate.
- Form tissues.
- Form organs.
- Become increasingly specialized.
From One Cell to Many
A fertilized egg begins as a single cell.
Through mitosis, this cell divides repeatedly.
One cell becomes:
2 cells → 4 cells → 8 cells → many cells
These cells eventually become organized into tissues and organs.
The instructions controlling much of this process are contained within the organism's DNA.
Because related species inherited many genes from common ancestors, similarities in their developmental processes can provide evidence of evolutionary relationships.
Comparing Embryos
Comparative embryology examines development in different organisms.
Vertebrate embryos are particularly useful for comparison.
Vertebrates include:
- Fish.
- Amphibians.
- Reptiles.
- Birds.
- Mammals.
Despite the large differences between adult members of these groups, their embryos share several important developmental features.
These similarities reflect the shared evolutionary history of vertebrates.
Early Developmental Similarities
During development, vertebrate embryos share several broad features.
Examples include:
- A basic body axis.
- A developing nervous system.
- A post-anal tail during some developmental stages.
- Pharyngeal arches.
- Segmented structures called somites.
- Similar early patterns of organ development.
The structures may later develop into very different adult forms.
The Basic Vertebrate Body Plan
Vertebrates share a basic body organization.
During development, structures appear in corresponding positions.
For example:
- The nervous system develops along the dorsal side.
- The digestive system develops through the body.
- Repeated blocks of tissue called somites appear.
- Pharyngeal arches develop in the head and neck region.
These similarities reflect inherited developmental patterns.
Pharyngeal Arches
Vertebrate embryos develop structures called pharyngeal arches in the head and neck region.
These structures develop differently in different vertebrate groups.
In fish, structures associated with the pharyngeal region contribute to structures supporting the gills.
In mammals, including humans, related embryonic structures contribute to parts of the:
- Jaw.
- Ear.
- Throat.
- Neck.
The important point is that corresponding embryonic structures can develop into different adult structures.
Humans Do Not Have Embryonic "Fish Gills"
A common oversimplification is to say that human embryos have gills.
This is inaccurate.
Human embryos develop pharyngeal arches, which are structures shared in the development of vertebrates.
They do not function as fish gills.
The similarity is important because the structures have related developmental origins, not because a human embryo passes through a fish stage.
The Embryonic Tail
Many vertebrate embryos develop a post-anal tail, meaning that part of the body extends beyond the developing anus.
In many vertebrates, this develops into a functional adult tail.
In humans, much of the embryonic tail regresses during development.
The remaining structures contribute to the coccyx, or tailbone.
The presence of corresponding developmental structures among vertebrates provides evidence of shared ancestry.
Somites
Somites are repeated blocks of embryonic tissue that form along the developing body.
They contribute to structures including:
- Vertebrae.
- Skeletal muscles.
- Parts of the skin.
Somites appear during the development of many vertebrates.
Their shared pattern provides another example of similarities in vertebrate development.
Similarity Does Not Mean Identical
Embryos of different species are not identical.
Differences can exist from very early stages of development.
However, related species often share developmental patterns and corresponding structures.
Therefore, embryological evidence should not be described as:
"All vertebrate embryos are the same."
A better statement is:
Vertebrate embryos share important developmental features because they inherited developmental programs from common ancestors.
Development Changes Over Time
As development proceeds, embryos become increasingly specialized.
For example, early vertebrate embryos may share several recognizable structural patterns.
Later:
Fish embryos
→ develop characteristics suited to aquatic life.
Bird embryos
→ develop bird-specific structures.
Mammalian embryos
→ develop mammalian characteristics.
The shared structures are modified differently as development continues.
Comparing Developmental Stages
When scientists compare embryos, they must compare equivalent developmental stages.
Simply comparing embryos of the same age in days can be misleading because different species develop at different rates.
Scientists examine features such as:
- Number of somites.
- Limb development.
- Organ development.
- Body shape.
- Developmental landmarks.
This allows more meaningful comparisons between species.
Shared Developmental Features
Examples of developmental features that may be compared include:
- Pharyngeal arches.
- Somites.
- Limb buds.
- Tails.
- Developing eyes.
- Nervous systems.
- Hearts.
- Body segmentation.
Scientists examine how these structures:
- Appear.
- Develop.
- Change.
- Become modified.
Patterns of similarity and difference can help reconstruct evolutionary relationships.
Limb Development
Tetrapods — vertebrates descended from ancestors with four limbs — share important developmental processes involved in forming limbs.
Early limbs begin as limb buds.
These structures can later become:
- Human arms.
- Bird wings.
- Whale flippers.
- Dog legs.
The adult structures may look very different, but similarities in their development support their homologous relationship.
Embryology and Homologous Structures
Comparative anatomy identifies homologous structures in adults.
Embryology can help explain how those structures are related.
For example:
Human arm
Bat forelimb
Whale flipper
all develop using related developmental pathways inherited from common vertebrate ancestors.
Embryology and comparative anatomy therefore provide complementary evidence for evolution.
Embryology and Common Ancestry
Evolution predicts that related organisms inherit genes and developmental processes from their ancestors.
Therefore:
Common ancestor
→ descendants inherit developmental genes
→ developmental patterns are modified over generations
→ related species retain some similarities
→ embryological similarities can reveal evolutionary relationships.
The closer the evolutionary relationship, the more developmental similarities scientists may expect to find, although the pattern is not always simple.
Developmental Genes
Embryonic development is controlled by networks of genes.
Some developmental genes are extremely ancient and are shared across very different groups of animals.
One important group is the Hox genes.
Hox genes help determine where different body structures develop along the body axis.
Related Hox genes occur in many animals.
The widespread conservation of developmental genes provides molecular evidence supporting common ancestry.
Evolution Can Modify Development
Evolutionary changes can occur when mutations affect:
- When a developmental gene becomes active.
- Where it becomes active.
- How strongly it is expressed.
- How long it remains active.
Small changes in developmental processes can sometimes produce substantial differences in adult anatomy.
This helps explain how organisms with similar developmental foundations can eventually develop very different adult forms.
Worked Example: Vertebrate Forelimbs
Consider:
- A human.
- A bat.
- A whale.
Their adult forelimbs perform very different functions.
However, each begins development as a limb bud.
Related developmental processes produce the same basic arrangement of limb bones.
Later development modifies:
- Bone lengths.
- Digit proportions.
- Overall shape.
The result is:
Human arm → manipulation
Bat wing → flight
Whale flipper → swimming
Shared development supports the conclusion that these structures are homologous.
Worked Example: Pharyngeal Arches
Consider fish and mammals.
Both develop pharyngeal arches as embryos.
In fish, structures associated with this region contribute to the gill-supporting apparatus.
In mammals, corresponding embryonic tissues contribute to structures in the:
- Jaw.
- Ear.
- Throat.
- Neck.
The structures develop differently but originate from related embryonic structures.
This pattern supports common ancestry.
Embryology and Evolution
Embryological evidence supports evolution because evolutionary history can leave traces in developmental processes.
A useful sequence is:
Common ancestry
→ inherited developmental genes
→ shared early developmental structures
→ evolutionary modification
→ different adult structures.
Embryology therefore helps scientists understand both similarities and differences among organisms.
Evolution Does Not Mean Embryos Repeat Evolutionary History
An older idea suggested that an embryo literally passes through the adult stages of its evolutionary ancestors.
For example:
fish stage → reptile stage → mammal stage
This is not an accurate description of development.
Human embryos do not become fish and then reptiles before becoming human.
Instead, related organisms share inherited developmental processes that have themselves evolved.
Developmental Similarities Can Reveal Hidden Relationships
Adult organisms may become extremely different through evolution.
Their embryos can sometimes reveal underlying similarities that are less obvious in adults.
For example, whales have highly modified bodies adapted for swimming.
Their developmental patterns still reveal their relationship to other mammals.
This makes embryology useful for studying evolutionary relationships.
Embryology and Whale Evolution
Modern whales do not have functional hind limbs for walking.
However, whale embryos temporarily begin developing hind-limb buds.
These structures later regress.
This developmental evidence is consistent with fossil and anatomical evidence showing that whales evolved from ancestors with functional hind limbs.
Development Can Preserve Evolutionary Clues
Some structures appear during embryonic development but are:
- Reduced.
- Modified.
- Reabsorbed.
- Incorporated into other structures.
These developmental patterns can provide clues about ancestry.
However, scientists interpret them alongside other evidence rather than treating every temporary embryonic structure as a direct picture of an ancestor.
Comparing Closely Related Species
Closely related species often share many developmental characteristics.
For example, mammals share developmental features associated with:
- Vertebrate body organization.
- Limb formation.
- Nervous system development.
- Organ formation.
Differences develop as species-specific genes and regulatory processes influence later development.
Comparing More Distantly Related Species
More distantly related organisms may still share basic developmental genes and processes.
For example, very different animals can share genes involved in:
- Body organization.
- Limb or appendage development.
- Eye development.
- Nervous system development.
These deep similarities provide evidence that modern animal groups inherited parts of their developmental systems from ancient ancestors.
Embryology and DNA Evidence
Modern embryology is closely connected to genetics.
Scientists can compare:
- Embryonic structures.
- Developmental genes.
- Gene expression.
- DNA sequences.
If two species share similar developmental structures and related genes controlling those structures, the combined evidence can strongly support an evolutionary relationship.
Embryology and Fossil Evidence
Fossils show changes in anatomy across geological time.
Embryology shows how anatomical structures develop.
Together they can provide a more complete evolutionary picture.
For example, whale evolution is supported by:
Fossils
→ show increasingly aquatic ancestors with changing hind limbs.
Comparative anatomy
→ reveals homologous mammalian structures.
Embryology
→ shows temporary hind-limb development.
DNA
→ confirms that whales are mammals closely related to particular groups of hoofed mammals.
Independent evidence can therefore support the same evolutionary explanation.
Inferring Evolutionary Relationships
When using embryological evidence, scientists ask:
- Which structures appear?
- When do they appear?
- How do they develop?
- Which developmental genes are involved?
- Are the structures homologous?
- How does the evidence compare with DNA and fossil evidence?
Species sharing detailed developmental patterns may share inherited developmental mechanisms from a common ancestor.
Worked Example: Two Unknown Species
Suppose Species A and Species B have very different adult appearances.
Scientists study their embryos and discover that both develop:
- Similar limb buds.
- Corresponding pharyngeal arches.
- Similar somite patterns.
- Similar developmental genes.
These similarities suggest that the species may share evolutionary ancestry despite their different adult appearances.
Scientists would then compare genetic and anatomical evidence to test this hypothesis.
Developmental Timing
Evolution can change not only what develops but also when development occurs.
Changes in developmental timing can affect:
- Body size.
- Limb proportions.
- Skull shape.
- Sexual maturity.
- Adult appearance.
Changes in the timing or rate of development are sometimes called heterochrony.
Such changes can contribute significantly to evolutionary differences between species.
Embryology and Natural Selection
Natural selection can influence developmental characteristics when genetic variation affects development.
Suppose a mutation changes the timing of limb development.
If the resulting adult structure increases reproductive success:
Developmental variation
→ different adult characteristic
→ reproductive advantage
→ greater inheritance of the genetic variant
→ population changes over generations.
Evolution can therefore occur partly through changes in developmental processes.
Why Embryological Evidence Is Powerful
Embryology provides a different kind of evidence from adult anatomy.
Adult structures show the final result of development.
Embryology reveals how those structures form.
Shared developmental pathways can reveal relationships that may not be obvious from adult appearance alone.
Limitations of Embryological Evidence
Embryology must be interpreted carefully.
Important limitations include:
- Embryos of different species are not identical.
- Development occurs at different rates in different species.
- Similar structures can become very different adult structures.
- Development itself evolves.
- Superficial resemblance alone does not establish ancestry.
- Equivalent developmental stages must be compared.
Scientists therefore combine embryology with other evidence.
A Useful Embryology Comparison Framework
When comparing embryos, consider:
Developmental stage
Are equivalent stages being compared?
Shared structures
What features occur in both embryos?
Development
What do these structures become?
Developmental genes
Are similar genetic pathways involved?
Evolutionary interpretation
Could the similarities have been inherited from a common ancestor?
Supporting evidence
Do fossils, anatomy, and DNA support the same relationship?
Common Mistakes
Saying Embryos of Different Vertebrates Are Identical
They share important features but are not identical.
Saying Human Embryos Have Fish Gills
Human embryos have pharyngeal arches, not functioning fish gills.
Saying Humans Have a "Fish Stage"
Humans develop as humans throughout development. Shared embryonic structures reflect common ancestry.
Assuming Similar Embryos Prove a Direct Ancestor-Descendant Relationship
Similarities indicate shared evolutionary history, not necessarily that one modern species descended from another modern species.
Comparing Embryos Only by Age
Different species develop at different rates, so equivalent developmental stages should be compared.
Assuming Development Perfectly Replays Evolution
Embryonic development does not literally repeat the adult forms of evolutionary ancestors.
Using Embryology Alone
Embryological evidence is strongest when combined with genetics, comparative anatomy, fossils, and other evidence.
Check Your Understanding
1. Define embryology.
2. What is an embryo?
3. Why can embryonic development provide evidence for evolution?
4. Name four developmental features that can be compared among vertebrate embryos.
5. What are pharyngeal arches?
6. Why is it inaccurate to say that human embryos have fish gills?
7. What is a post-anal tail?
8. What are somites?
9. Explain why embryos of different vertebrates share some developmental characteristics.
10. Why do embryos become increasingly different as development proceeds?
11. Explain how limb development provides evidence for common ancestry among tetrapods.
12. How can embryology support evidence from comparative anatomy?
13. What are Hox genes?
14. Why are shared developmental genes important evidence for evolution?
15. Explain how changes in developmental gene activity could contribute to evolution.
16. Why should equivalent developmental stages be compared rather than simply embryos of the same age?
17. Explain how whale embryology provides evidence about whale ancestry.
18. Why is it incorrect to say that embryonic development completely repeats evolutionary history?
19. Two species look very different as adults but have similar embryonic structures and developmental genes. What might this suggest?
20. Explain how embryological, fossil, anatomical, and genetic evidence can work together to support common ancestry.
Key Terms
- Embryology – study of the development of organisms during embryonic stages.
- Embryo – organism during an early stage of development.
- Zygote – cell produced by fertilization.
- Differentiation – process through which cells become specialized.
- Common ancestor – ancestral population from which different evolutionary lineages descended.
- Pharyngeal arches – embryonic structures in the head and neck region shared among vertebrates and modified into different adult structures.
- Post-anal tail – embryonic or adult body structure extending beyond the anus.
- Somites – repeated blocks of embryonic tissue contributing to structures including vertebrae and skeletal muscles.
- Limb bud – early embryonic structure from which a limb develops.
- Homologous structures – structures sharing an evolutionary origin.
- Hox genes – developmental genes involved in determining body organization along the body axis.
- Heterochrony – evolutionary change involving the timing or rate of developmental processes.
- Comparative embryology – comparison of developmental patterns among different organisms.
- Common ancestry – evolutionary relationship resulting from descent from shared ancestral populations.
Key Takeaways
- Embryology is the study of how organisms develop during early life.
- Embryos of different species can share important developmental features.
- Vertebrate embryos share aspects of a common basic developmental pattern.
- Shared features include pharyngeal arches, somites, limb buds, and post-anal tails during particular stages.
- Embryos of different species are similar in some ways but are not identical.
- Human embryos do not have functioning fish gills; they develop pharyngeal arches.
- Corresponding embryonic structures can develop into very different adult structures.
- Shared developmental patterns can provide evidence of common ancestry.
- Related organisms inherit developmental genes and processes from their ancestors.
- Hox genes are important examples of ancient developmental genes shared across many animals.
- Evolution can modify the timing, location, and activity of developmental genes.
- Adult homologous structures often share related developmental pathways.
- Embryological evidence can reveal relationships that are difficult to see from adult appearance alone.
- Development does not literally repeat evolutionary history.
- Whale hind-limb development provides an example of embryological evidence consistent with fossil and anatomical evidence.
- Equivalent developmental stages must be compared carefully.
- Embryology is strongest as evolutionary evidence when combined with comparative anatomy, fossils, genetics, and other independent evidence.