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Using DNA sequences and early developmental stages to map the history of life.
Did you know that in the first few weeks of life, you had a tail and structures that look remarkably like fish gills? How can our bodies and our DNA act as a 'time machine' to reveal ancestors from millions of years ago?
Every living thing uses the same genetic code. By comparing the order of nucleotides in DNA or the sequence of amino acids in proteins, scientists can see how closely related two species are. Think of it like a game of 'telephone': the longer the message travels (the more time passes), the more 'typos' or mutations accumulate. If two species have very similar DNA, they shared a common ancestor recently. If their DNA is very different, they branched apart a long time ago. For example, humans and chimpanzees share about of their DNA, indicating a very recent common ancestor in the tree of life.
Let's look at a short segment of the Cytochrome c protein (used in energy production) for three species: 1. Human: 2. Chimpanzee: 3. Dog:
Step 1: Compare Human to Chimpanzee. There are differences. Step 2: Compare Human to Dog. There is difference ( vs ). Conclusion: Humans are more closely related to chimpanzees than to dogs.
Quick Check
If Species A has 5 DNA differences from a human, and Species B has 25 differences, which species shared a common ancestor with humans more recently?
Answer
Species A, because fewer mutations (differences) suggest less time has passed since they branched off from a common ancestor.
Before animals are born, they go through early stages of development called embryos. Interestingly, the embryos of different vertebrates (animals with backbones) look almost identical in their earliest stages. All vertebrate embryos—including humans, chickens, and fish—possess a post-anal tail and pharyngeal pouches (which become gills in fish and parts of the ear/throat in humans). These shared structures are not used by all adults, but they exist in the embryo because they were inherited from a common ancestor. This is known as comparative embryology.
Consider a human embryo and a fish embryo at 'Stage 1' of development: 1. Both have a curved 'C' shape. 2. Both have a visible tail bud. 3. Both have slits in the neck area (pharyngeal pouches).
As they develop to 'Stage 3': 1. The fish develops fins and functional gills. 2. The human loses the tail and the pouches transform into jaw and inner ear bones.
This shows that while the adult forms are vastly different, the instructions for the early stages are still the same.
Quick Check
What are the two specific structures found in almost all early vertebrate embryos that suggest a common ancestor?
Answer
Pharyngeal pouches (or slits) and a post-anal tail.
A molecular clock is a technique that uses the mutation rate of biomolecules to deduce the time when two life forms diverged. If we know that a specific gene mutates at a relatively constant rate—say, one mutation every million years—we can count the differences between two species to estimate their 'split' date. If there are mutations of difference, and the rate is mutation per million years, we can calculate the time since they shared an ancestor using simple multiplication.
Scientists are studying two species of frogs separated by a mountain range. They find differences in a specific DNA sequence. They know this gene mutates at a rate () of mutation every million years.
Conclusion: The two species likely shared a common ancestor million years ago.
Which of the following would provide the strongest evidence that two species are very closely related?
Why do human embryos have pharyngeal pouches (slits) even though humans do not have gills?
A molecular clock relies on the assumption that mutations in certain genes occur at a relatively constant rate over time.
Review Tomorrow
In 24 hours, try to explain to a friend why a chicken embryo and a human embryo look almost identical in their first few days of development.
Practice Activity
Research the 'Cytochrome c' protein online and find out which animal's sequence is most similar to a human's: a Rhesus monkey or a Yeast cell.