Do evolutionists believe that we all have a common human ancestor?

The short answer is Yes. There was a common male ancestor, and a common female ancestor as well, but they never saw each other -- they lived 250,000 years apart. Dawkins goes into considerable detail on this, and I recommend it.

The role of horizontal transfer of genes is low. How low is still debated (I think it's towards the high end of the range -- extremely uncommon). Many of the mutations (including chromosomal rearrangements) probably occurred once and were transfered to multiple offspring. The major mode of blending these often one-time events is recombination by sexual reproduction followed by natural selection. Those who received multiple favorable mutations lived to pass them on.

Balanced translocations are fairly common. A translocation with fusion of chromosomes 13 and 14 occurs in about one person in 1300. If any isolated human population had the translocation in one or two of the founders, it could have led to a population of humans that were a different species.

Yes, science has determined that we humans are all related by a common ancestor. Of course, some of us have a more recent common ancestor than others, but overall there is a point where all humans are related. I think you have a basic misunderstanding of evolution. It is better described as a 'bush' than as a 'tree'. There is not a ladder upward, but a expanding sequence, or brances, of relationships. Some of those branches die out (go extinct) but not all.

Check out the www.talkorigins.org site for more information on evolution.

Anthropologists dissagree on this point. Some subscribe to the African Replacement model, which states that modern humans evolved in Africa and then spread to the other continents, out-competing and replacing the other hominins that lived there without interbreeding. Another hypothesis is the "Multi-Regional Evolution" hypothesis, whereby modern humans evolved from other hominins in Africa, Asia, Europe and Indonesia seperately. there are combinations of these hypotheses, too... such as African Replacement with Hybridization and African Replacement with Assimlation... which follow that modern human-ish hominins came out of Africa and interbread with other populations of hominins with varying degrees of inheritence.

I think that the author of this question is mistaken in thinking that evolution works in such a way were one could give birth to an offspring that is of a different species than oneself. Mutations (that produce viable, fertile offspring) are never severe enough to cause a change that would distinguish an offspring enough from its parents to call it a new species. it is not individuals that evolve, or even single lineages that evolve and then out-compete all other lineages. Rather it is populations (or more precisely gene pools) that evolves. Evolution can be thought of as a change in allele frequency thoughout a popluation... as allele frequency changes (due to natural selection for genotypes that produce greater fitness) the ratio of phenotypes changes, and as this change becomes greater and greater, (and though isolation of popluations) we new "species" emerge. I put "species" in quotes because it's important to understand that there is not one, clear defintion of a species and that to apply it to ridgedly (but saying "when did one species split and become two") is misleading and makes it hard to understand evolutionary processes.

also, "for evolution to be true, there has to be a series of mutations resulting in more complex organisms." this is not true. Evolution does not lead to more complex organisms, it results in organisms that are better adapted to their envrionment and have higher fitness... this does not nesecarily mean that something has to become more complex... in some cases it is advantages to be less "complex" as a consequence of envrionmental change. (but "complexity" is also a tricky definition.)

the key point is that evolution doesn't work on individual lineages in isolation. If a mutation occurs and it is benefitial (that is allows that individual to reproduce more effectively) then that individual has a greater chance to pass on this gene to it's decedents, theyby increasing the frequency of this gene in the gene pool... if the decedents are sucessful in their reproducing, as well, they will also increase the frequency of this gene and over time it may spread into the population. in this way the genetic makeup of a population changes over time... and "change over time" is the defintion of evolution.

Yes, and you've also hit on one of the major problems with macro-evolution. If you have one genetic mutation with a unique number of chromosomes (speciation) then what does the creature successfully breed with to pass on this varying chromosome number? Some folks today call this the trisomal paradox.

The only way to successfully account for speciation is for entire populations to mutate identically at once so they have the same number of chromosome pairs and at levels great enough to overcome inbreeding. When you combine this with the fact that this scenario must take place for every species on the planet, the odds of it occurring become astronomically large, even given huge amounts of time and random chance.

No matter what you believe, KNOW THIS:

No one alive ever had an ancestor who failed to succeed in breeding.

No matter what you believe, know that the only way our ancestors could breed was through sex.

No matter what you believe, in order to breed our ancestors needed a mate of the opposite sex.

No matter what you ask, that's the way it was.

No matter what our ancestors were, we descended from them.

what do you mean y horizontal and vertical?