Have you ever wondered how different living things are connected? Let's talk about something called "common ancestry" which is basically how all living things share a common ancestor. Scientists use a variety of evidence to support this idea, like comparing DNA and studying fossils. Understanding common ancestry also helps us understand how evolution works. So, even though we might look different on the outside, we all have a lot in common on the inside!

What Does Common Ancestry Mean?

Common ancestry is the idea that all living things have descended from one common ancestor, resulting in the formation of new species through evolution. When two or more species share a recent common ancestor, they are considered closely related. In contrast, when two or more species do not share a recent common ancestor, they are considered more distantly related. This is because scientists believe that every life form can be traced back to one common ancestor, making it the central concept in Darwin's book, On the Origin of Species. By studying common ancestry, we can better understand how different species are related and how they have evolved over time.

What is the theory of common ancestry?

The theory of common ancestry suggests that all living things came from one "universal common ancestor." According to Darwin, similarities among species could indicate that they are related and can be traced back to a common ancestor. For instance, Darwin believed that the various finch species found on the Galapagos Islands came from one original species that colonized the islands millions of years ago. As the populations of the ancestor species moved from island to island, they adapted to different environments and evolved into many different descendants. By studying common ancestry, we can better understand how species are related and how they have evolved over time.

Darwin came up with this hypothesis from his observation that the finches had very similar traits and were only different in terms of beak shapes and feeding habits that allowed them to adapt to their specific environment (Fig. 1).

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The example of the finch species on the Galapagos Islands demonstrates how ancestral species can branch out into new species through evolution. As we look back in time, we can trace species back to smaller and smaller groups of common ancestors. The theory of common ancestry suggests that all life forms descended from one "universal common ancestor," as Darwin proposed. He believed that all organic beings that have ever lived on Earth likely descended from one primordial form. This "universal common ancestor" is often referred to as LUCA (Last Universal Common Ancestor) and is believed to have lived between 3.5 and 4.5 billion years ago. While LUCA was not the first living organism, it is considered the earliest ancestor of all currently living species. By studying LUCA and the theory of common ancestry, we can gain a better understanding of the origins and evolution of life on Earth.

Is There Evidence of Common Ancestry of Life?

Similarities shared by organisms, and patterns in the fossil record, provide evidence of common ancestry. This section will discuss homology and fossils as evidence of common ancestry.

Similarity Resulting From Common Ancestry Is Known as Homology

The existence of similar traits and features among different species can provide evidence for common ancestry. Homology refers to the similarities in traits and features between organisms that are inherited from a common ancestor. By studying homology, we can infer how different species are related. There are three types of homology: morphological, molecular, and developmental homology.

Morphological homology refers to similarities in the physical structure and form of organisms. For instance, mammals can be classified as monotremes, marsupials, and placentals based on similar features. Monotremes, such as platypuses, are mammals that lay eggs. Placentals, like rodents, dogs, and whales, are mammals with placentas, a temporary organ that connects the embryo to the mother's uterus. Marsupials, like kangaroos, wombats, and koalas, use external pouches to raise their newborn offspring. Organisms within each group, monotremes, placentals, and marsupials, are classified as such because they share similar structures and can be traced back to a common ancestor.

Studying homology can help us understand the evolutionary history of different species and their relationships to one another. By examining the similarities and differences among organisms, we can reconstruct the evolutionary tree of life and gain insights into how life has diversified and evolved over time.

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In molecular homology, similarities can be observed in the genes or DNA sequence of the species. These similarities could result in similar observable traits, but this is not always the case; there are instances where two or more species have major morphological differences but have nearly identical genes. For this reason, genetic information like DNA is important evidence of common ancestry.

For example, Hawaiian silversword plants across the islands of Hawaii look very different, but their genes are very similar (Figs. 3 & 4).

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Additionally, all life forms share the same genetic material. From bacteria to humans, all life forms have DNA and its mechanism for replication and expression, suggesting that all species came from a very distant common ancestor. In developmental homology, similarities can be observed in particular developmental stages of the organisms. For example, all vertebrate embryos (even humans!) have gill slits and tails that disappear by the time of birth. We can infer that all vertebrates can be linked to a common ancestor (Fig. 5).

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Patterns in the Fossil Record Provide Evidence of Common Ancestry

Fossils are extremely important for understanding the history of life on Earth. They are preserved remnants or traces of organisms from past geologic ages, and they provide evidence of how gradual changes in the features of pre-existing organisms led to the formation of new species over time. By examining fossils, we can learn about the morphology, behavior, and ecology of extinct organisms, as well as their relationships to living species.

Fossils can also help us trace the origins of modern organisms. For instance, the fossil record shows that birds evolved from dinosaurs. The feathers of some dinosaurs evolved into wings over time, allowing them to take flight and eventually giving rise to modern birds. By examining the fossils of these early birds and comparing them to those of their dinosaur ancestors, we can link the traits of modern birds to those of their prehistoric ancestors.

Fossils also provide a unique window into the past, allowing us to study the diversity of life that existed long before humans appeared on the scene. By examining fossils, we can learn about the evolution of different groups of organisms, as well as their interactions with each other and with their environments. Overall, fossils are an invaluable tool for understanding the history of life on Earth and the processes that have shaped the diversity of organisms we see today.

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By observing similarities among species and patterns in the fossil record, we can infer how species are related, where they originated, and how their features changed in evolution. Inferences about the common ancestry of different species can be visualized through phylogenetic trees (Fig. 8).

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Most Similarities in Morphology, Fossils, and Embryos Are Results of shared DNA - an Outcome of Common Ancestry.

You are correct! The similarities in morphology, fossils, and embryos of organisms all ultimately come down to shared DNA or genetic information. The genetic information encoded in an organism's DNA determines its observable traits and how it interacts with the environment. Organisms that share similar DNA are likely to have similar traits, even if they are separated by millions of years of evolution.

Morphological and developmental similarities among organisms are expressions of shared DNA. For example, the similar forelimb structure of mammals and the wings of birds are both derived from the same ancestral structure, modified over time to suit different functions. Similarly, the similarities in the development of vertebrate embryos reflect shared genetic information that has been conserved over millions of years of evolution.

Fossils also provide evidence of shared genetic information. By studying the morphology of fossils, we can infer the traits of ancestral organisms and trace the evolution of those traits over time. Similarities in fossils can be used to reconstruct the evolutionary relationships between different groups of organisms and to understand the patterns of diversification and extinction that have shaped the history of life on Earth.

Overall, the similarities in morphology, fossils, and embryos of organisms all reflect the shared genetic information that is the immediate outcome of common ancestry. By studying these similarities, we can gain insights into the evolutionary history of life on Earth and the processes that have shaped the diversity of organisms we see today.

How Does Common Ancestry Provide Evidence for Evolution?

Common ancestry is a fundamental concept in evolution that explains how new species emerge from pre-existing species over time. Through the process of natural selection, organisms with advantageous traits are more likely to survive and pass on those traits to their offspring. Over time, this can lead to the gradual accumulation of genetic differences between populations, eventually resulting in the formation of new species.

Common ancestry also helps us to understand the unity and diversity of life on Earth. All living organisms share a common ancestor, and the diversity of life that we see today is the result of millions of years of evolution and adaptation to different environments. By studying the similarities and differences between different groups of organisms, we can gain insights into the evolutionary relationships between them and the processes that have shaped their diversity.

Natural selection is a key mechanism of evolution that drives the gradual and cumulative change in the heritable traits of a population of organisms over time. Individuals with advantageous traits are more likely to survive and reproduce, passing on those traits to their offspring. Over many generations, this can lead to the evolution of new adaptations and the formation of new species.

Overall, common ancestry, natural selection, and evolution are all interconnected concepts that help us to understand the diversity of life on Earth and the processes that have shaped it over time.

Common Ancestry - Key takeaways Common ancestry means having descended from one ancestor. Sharing a recent common ancestor means that two or more species are closely related. Not having a recent common ancestor means that two or more species are distantly related. By observing similarities among species and patterns in the fossil record, we can infer how species are related, where they originated, and how their features changed in evolution. Similarities among species are called homologies. There are three major types of homology: Morphological homology: similar structure and form Molecular homology: similar genes or DNA sequence Developmental homology: similar developmental stages Fossils show how gradual changes in the features of pre-existing organisms led to the formation of new species over time. In general, the more similarities organisms share, the more closely related they likely are. Most of these similarities are results of shared DNA The theory of common ancestry holds that all life forms descended from one "universal common ancestor".

Common Ancestry

Structures that share a common ancestry are

Structures that share a common ancestry are homologous

What does common ancestry mean?

Common ancestry (also referred to as common descent) means having descended from one ancestor. 

How can patterns be used to identify common ancestry?

Similarities shared by organisms, as well as patterns in the fossil record, provide evidence of common ancestry. In general, the more similarities shared by organisms, the more closely related they likely are. 

How does common ancestry provide evidence for evolution?

Common ancestry provides evidence for evolution because it shows that new species emerge from pre-existing species, implying that life forms change over time. Common ancestry also shows that one ancestral population could diversify into many descendant species with modifications that are more suited to their present environment. 

What indicates common ancestry?

Similar traits and features among various species can provide evidence of common ancestry. In general, the more similarities shared by organisms, the more closely related they likely are. These similarities can be observed in the morphology, genes, and developmental stages of organisms. Fossils also show common ancestry by showing  how gradual changes in the features of pre-existing organisms led to the formation of new species over time.

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