All life shares DNA, ribosomes, the genetic code, and glycolysis. All eukaryotes add membrane-bound organelles, linear chromosomes, and introns.
Every living cell stores genes in DNA, builds proteins on ribosomes, and reads nearly the same genetic code. Chemistry didn't require those exact details. Their sharing points to one thing: all life descends from a common ancestor.
Key terms
- Common ancestry — lineages descending from the same ancestral population. Trace any two living species back far enough and they meet.
- Conserved sequence — DNA, RNA, or protein that has barely changed across lineages, because selection removes most changes to it.
- Genetic code — the table matching codons to amino acids. Nearly every organism uses the same one, though many other tables would work.
- Homology — similarity due to shared ancestry. It shows up in bones, in embryos, and in DNA and protein sequences.
- Convergent evolution — separate lineages evolving similar forms because they face similar demands. Similar shape, separate histories.
What shared features tell you
- All three domains store genes in DNA, copy them into RNA, and build proteins on ribosomes. The rRNA at the ribosome's working core matches closely across all three.
- Nearly all life reads the same code. Reassigning one codon would alter nearly every protein at once, so the code has stayed almost fixed since the common ancestor.
- Glycolysis runs in all three domains. Its enzymes grow less similar the more distant two organisms are: descent with modification.
- All eukaryotes share a further set: membrane-bound organelles, linear chromosomes, and genes interrupted by introns. That set marks a more recent ancestor shared by eukaryotes alone.
- Vertebrate embryos all form pharyngeal arches and a post-anal tail. The arches later become gill supports in fish and jaw, ear, and throat parts in mammals.
Why it matters
Questions here name a shared feature and ask what it shows. Sort it by who has it. In all three domains → all life shares an ancestor. In every eukaryote → eukaryotes share a more recent one. When body shape and a slowly changing gene disagree, trust the gene. Shape tracks the environment; conserved sequences track history.
Exam traps
| Trap | Actually |
|---|
| Sharks, dolphins, and ichthyosaurs share a streamlined shape, so they share a recent ancestor. | They're a fish, a mammal, and a reptile. Moving fast through water selected the same shape in three separate lineages. |
| Life shares one genetic code because it's the only one that works chemically. | Many codes would work, since tRNAs and their enzymes set the pairings. The code is shared because it was inherited, then stayed nearly fixed. |
| Glycolysis is essential, so each lineage evolved it on its own. | Being essential keeps a pathway conserved, not reinvented. Similarity that fades steadily with distance is the signature of one shared origin. |
| Embryos pass through their ancestors' adult forms. | Early embryos look alike because they share one inherited program. A mammal embryo's pharyngeal arches become jaw, ear, and throat parts, never gills. |
Worked example
Researchers culture a new microbe from a deep-sea vent. It has no nucleus and one circular chromosome, and its genes lack introns. It reads the standard genetic code, runs glycolysis, and its ribosomal RNA carries the core sequence found in all known life. A student claims it might descend from a separate origin of life. Evaluate the claim.
- Sort the clues by who shares them. No nucleus, a circular chromosome, no introns: these mark it as a prokaryote. They say nothing about a separate origin.
- Standard code, glycolysis, conserved rRNA core: all three domains share these. A separate origin would be very unlikely to land on the same arbitrary code and the same rRNA sequence.
- So it belongs on the same tree as everything else, as a bacterium or an archaeon.
The claim fails. The shared code and rRNA core mean it descends from the same common ancestor as all known life. Missing eukaryote features make it a prokaryote, not an outsider.
Universal features link all three domains; features shared by every eukaryote mark one branch. The near miss reads 'lacks eukaryote features' as 'unrelated to everything'. But bacteria lack a nucleus too, and they share our code and our ribosome core.